3-D Seismic Examples from Central Lake Maracaibo, Maraven's Block I Field, Venezuela

Martin H. Link, Christopher K. Taylor, Nicolas G. Munoz J., Emilio Bueno, and Pedro J. Munoz, 

Search and Discovery Article #20002 (1999)  

CHAPTER 7 Martin H. Link¹ , Christopher K. Taylor², Nicolás G. Muñoz J.³, Emilio Bueno, and Pedro J. Muñoz (Maraven,S. A., Caracas, Venezuela.)

¹Landmark Graphics, Caracas, Venezuela. Current address: P.O. Box 62, Tidewater OR 97390.
²Geco Parkla, Caracas, Venezuela.
³S. A. Consultores CSC, Venezuela

(Adapted for online presentation from article of same title by the authors given above published in P. Weimer and T. L. Davis, eds., AAPG Studies in Geology No. 42 and SEG Geophysical Developments Series No. 5, AAPG/SEG, Tulsa, p. 69-82.)

ABSTRACT

The structural and stratigraphic framework of Maraven's Block I was re-interpreted using 3-D seismic and existing data as part of an evaluation of the remaining oil potential. More than 1800 MMBO have been produced from Block I in the past 40 years, mainly from structural traps. In order to maintain production levels, it has become increasingly important to define the seismic stratigraphic framework for the area and to accurately locate faults and stratigraphic pinchouts.

The dominant structures are the Icotea fault, its conjugate fault system, and the Eastern Boundary fault. The most prominent fault is the NE-striking Icotea fault, which subdivides the area into two main structural blocks, a graben in the West Flank and a horst in the East Flank. The Icotea fault is a highly complex fault zone with a long history of deformation. It is a nearly continuous fault zone with both vertical and lateral offsets and is locally inverted. Along the eastern flank of the Icotea, prominent reverse-fault bounded upthrown blocks, called the Attic, have developed. Along the western flank, contraction has re-activated listric faults into reverse and thrust faults. Major northwest-striking normal faults delineate a large paleoarch that occurs in the south-center of the East Flank. This phase of faulting produced small horst and graben blocks bounded by normal faults that dip to the northeast and southwest. The Eastern Boundary fault is subparallel to the Icotea fault and is an east-dipping normal fault that has been locally inverted and occurs in a synclinal area of the block. Two play concepts, utilizing (1) horizontal wells in Attic and (2) vertical wells along the Eastern Boundary fault, were successfully tested during this study.

The stratigraphic section includes, from oldest to youngest, pre-Triassic basement rocks; the Jurassic graben-fill Quinta Formation; the Cretaceous Rio Negro, Cogollo Group, La Luna, Colon, and Mito Juan formations; the Paleocene Guasare Formation; the Eocene Misoa Formation; the Miocene La Rosa, Lagunillas, and La Puerta formations; and the Quaternary El Milagro Formation. Only the lower part of the Eocene Misoa Formation (C sands) is preserved in Block I, and most of the Eocene B sands and all of the Pauji were either eroded or not deposited in this area. The main reservoirs occur in the Eocene Misoa Formation and the basal Miocene Santa Barbara member of the Lagunillas Formation. Sedimentation occurred throughout the Eocene and was strongly influenced by tectonism. The Eocene section in the horst block is up to 760 m thick and is bracketed by two major unconformities. The upper angular unconformity places the basal Miocene Santa Barbara member (16-25 Ma) over the Eocene Misoa C sands (45-54 Ma). The lower disconformity (54 Ma) occurs at the top of the Paleocene Guasare Formation. In between, eight seismic sequences occur within the Eocene horst section. The adjacent stratigraphic sections east and west of the horst block are thicker than the East Flank section. The C sands in Block I form a retrogradational clastic sequence deposited as transgressive (70-80%), highstand (10-15%), and lowstand wedge and incised valley fill (10-15%) systems tracts with prominent marine-flooding surfaces separating these systems tracts. The main reservoirs are thick-bedded transgressive sandstone deposits.

INTRODUCTION

The use of 3-D seismic data to interpret old producing fields has become very important in field development. When integrated with well, core, and production data, 3-D seismic data provide the best tool to determine the stratigraphic and structural framework for an area. Maraven, S.A., one of Venezuela's three national oil companies, has been using 3-D seismic data to locate new wells by accurately defining faults and stratigraphic terminations/pinchouts and by developing new geologic models to better develop their existing fields. In addition, the mapping of seismic horizons and amplitudes combined with time and horizon slices have been used to determine reservoir trends in fields. The Maracaibo Basin is an intermontaine basin today, lying between the Perija and Merida Andes in the northwest corner of Venezuela. Maraven's Block I is 1022 km and is in the north-central part of Lake Maracaibo along the Icotea fault (FIGURE 1). This field has more than 550 wells, which have produced more than 1800 MMBO during the past 40 years, mainly from structural traps. The Block I area is the southern continuation of the Lagunillas field and the northern part of the Lama field (Delgado, 1992). The purposes of these examples are to show the use of 3-D seismic data to (1) define the structure and stratigraphy of this Block by integrating older log, core, and production data, (2) to develop a seismic and sequence stratigraphic model for the Lower Eocene Misoa C sands in the East Flank, and (3) highlight two successful wells that were drilled using 3-D seismic technology. The 3-D seismic data allow, for the first time, the complex fault relationships of the Icotea and Eastern Boundary faults to be resolved and major sequence boundaries and systems tracts to be recognized. The 3-D seismic data were utilized to illustrate some of the various methodologies used to better visualize the reservoir at both a regional and a prospect scale.

METHODOLOGY

The structure and stratigraphy of Maraven's Block I were re-interpreted using 3-D seismic data integrated with well logs, cores, older 2-D seismic lines, and synthetic seismograms. Both Landmark and Charisma software with Sun workstations were used to construct maps, cross sections, and various 3-D graphic presentations. The Block I 3-D seismic survey was collected and processed during 1990-1991 and covers an area approximately 20.4 km long by 11.6 km wide (FIGURE 2). The shot lines were recorded perpendicular to the Icotea fault to ensure optimum imaging of the subsurface. Between 250 and 300 time-based logs were combined with the seismic data to correlate horizons. Eight velocity surveys and four synthetic seismograms were used to calibrate and adjust the data. Regional 2-D seismic lines were used to identify the dip and shape of the main fault traces, especially where the quality of the 3-D seismic data was diminished due to the edge effect of the survey. In addition, ten cored wells, containing more than 300 m of core, helped correlate lithofacies to well logs and calibrate different seismic facies.

Time horizons, structures, stratigraphic tops, sequence boundaries, and marine-flooding surfaces were mapped from seismic data in the area. Structures were defined using horizon and dip maps and fault polygons interpreted from seismic lines (FIGURE 2 and FIGURE 3). The stratigraphic framework was established by (1) correlating horizons from existing well tops using over 300 wells and by (2) extrapolating tops into areas with little well control. The sequence stratigraphy terminology follows Mitchum (1977), Vail (1987), Posamentier and Vail (1988), and Van Wagoner et al. (1990). Sequences, sequence boundaries, marine-flooding surfaces, systems tracts, and parasequences were interpreted and correlated for the Eocene section in the Block I horst. Seismic and wireline-log cross sections were used together to carefully identify sequence boundaries, marine-flooding surfaces, and faults.

STRUCTURE

Block I area has a long and complex structural history (Link et al., 1994). The area is on the eastern margin of a Jurassic rift basin that trends north-south (Bartok et al., 1981; Bartok, 1993; FIGURE 1A). This graben, containing redbeds of Jurassic La Quinta Formation, has been deformed and later eroded (FIGURE 1B and FIGURE 4). The dominant structures of Block I are the Icotea fault, its conjugate fault system, and the Eastern Boundary fault (FIGURES 1, 2 , 3 , 5 , 6 , 7 , 8 , 9, 11). These faults follow the older Jurassic basin trends. The most prominent fault is the northeast-striking Icotea fault, which subdivides the area into two main structural blocks, a graben in the West Flank and a horst in the East Flank (FIGURE 2 and FIGURE 5). The Icotea fault is a highly complex fault zone with a long history of deformation. Along the eastern flank of the Icotea, prominent reverse-fault bounded blocks, called the Attic, have developed. On the West Flank, contraction has re-activated normal faults into reverse and thrust faults. The Eastern Boundary fault has a trend similar to that of the Icotea and merges with it, farther to the north, outside Block I (FIGURE 2). This listric fault is downthrown to the east and was later re-activated as a reverse fault (Figures FIGURE 5 and FIGURE 6). Northwest-striking normal faults delineate a large paleoarch that occurs in the south-center of the East Flank (Figures FIGURE 2 and FIGURE 3). This arch contains smaller horst and graben blocks bounded by normal faults that dip to the northeast and southwest.

The Icotea fault system strikes northeast across the west-central part of Block I (FIGURE 2). This fault zone contains the main trace of the Icotea and is flanked by several normal and reverse faults that dip to both the east and the west (FIGURE 2, FIGURE 3, FIGURE 5, and FIGURE 6). The Icotea fault is a relatively linear, continuous narrow zone, about 0.5-1 km wide and more than 100 km long. Along the strike, this narrow fault zone has both anticlinal and synclinal structures (FIGURE 7, FIGURE 8, and FIGURE 11). Vertical displacement along the Icotea fault in the West Flank basin is about 1-2 km, and movement is down to the west (FIGURE 7, FIGURE 8 and FIGURE 11). The amount of transpressional slip on the Icotea fault system could not be determined in Block I. The estimated left slip of 0.8 km is based on offset surface topography and subsurface mapping (Lugo, 1992). Miocene compression created inversion locally along the Icotea and related faults with about 15 of clockwise rotation for the Icotea fault-bounded blocks (Lugo, 1992). The Icotea fault is the boundary between the horst and graben (FIGURE 2, FIGURE 3, and FIGURE 5).

The Icotea fault had been interpreted as a normal fault, strike-slip (Lugo, 1992, among others), thrust fault (Delgado, 1992), and inversion structure (Roberto et al., 1993). We believe that four separate faults occur in close proximity along the Icotea and each can be distinguished on seismic profiles. Each fault system is superimposed upon another: (1) early Eocene high-angle reverse fault(s), followed by (2) early Eocene normal faults, (3) a middle to late Eocene listric fault system with a detachment surface at the Cretaceous Colon level, and (4) late Eocene and early Miocene inversion-related reverse and thrust faults with a strike-slip component (i.e., Attic fault) (see FIGURE 4, FIGURE 5, FIGURE 6, and corresponding structural events [1]-[4] in FIGURE 5). We believe the Icotea fault originally formed as an east-dipping reverse fault (Icotea 1) (FIGURE 5). A west-dipping boundary fault (Icotea 2) developed later over the eastern edge of the Jurassic half-graben, directly overlying and overprinting the earlier reverse or ramp fault. A thick Eocene stratigraphic section was deposited on the downthrown side of the fault, forming the West Flank Basin (FIGURE 5, FIGURE 6, FIGURE 7, FIGURE 8). West-dipping listric normal faults (Icotea 3) formed during and after sedimentation, utilizing the older fault traces. Both antithetic and synthetic faults were associated with the listric faults, which sole out in the underlying Cretaceous Mito Juan/Colon and Paleocene shales (FIGURE 5, FIGURE 6 and FIGURE 8). Later during the early Eocene, transpression was caused by the oblique collision between the Caribbean and the South American plates. The Icotea and its listric fault system were re-activated as reverse and thrust faults (Icotea 4) with some strike-slip component caused by the clockwise rotation of the blocks (Bueno et al., 1993; Roberto et al., 1993). During inversion, west-directed backthrusts occurred during the late Eocene and again in the Miocene. Experimental work and field observation of inversion structures show that thrust and reverse faults commonly form and use existing extensional structures (Williams et al., 1989; McClay and Buchanan, 1991), especially where the original structures had a low-angle dip.

The Attic fault is the easternmost reverse fault of the Icotea system and, where present, always contains the East Flank section (FIGURE 2, FIGURE 3, FIGURE 5, FIGURE 6, FIGURE 7, FIGURE 8, FIGURE 9 and FIGURE 11). It consists of two segments: (1) a northern part, and (2) a southern part, separated by a saddle in the middle, where the Attic fault merges with the main trace of the Icotea fault (FIGURE 2, FIGURE 5, FIGURE 6, FIGURE 7, and FIGURE 9). The Attic fault verges to the east, and its vertical displacement is 50-100 m. The origin of these fault blocks is probably related to late-stage shortening, where the western corner of the horst block was thrust- or reverse-faulted over itself (FIGURE 5). The Attic fault may have originated as a west-directed synthetic fault of a listric fault system. During inversion, this fault was re-activated and thrust eastward. Northwest- and northeast-striking cross faults cut the Attic block into a series of smaller fault blocks. These faults are young and cut the Attic and Icotea faults as well as the Eocene unconformity. The use of 3-D seismic data allowed the complexities of the Attic fault blocks to be imaged for the first time.

A large paleoarch occurs in the south-central part of the East Flank of Block I and is characterized by northwest-striking normal faults (FIGURE 2, FIGURE 3, FIGURE 7, FIGURE 8, FIGURE 9, and FIGURE 10). The paleoarch trends northwest and is 6-10 km wide and at least 60 km long, extending into the Lagoven area to the east. This arch may be the northwesternmost continuation of the Merida Arch (Lugo and Mann, 1995). The arch does not appear to extend across, or to have been offset along, the Icotea fault. The younger Icotea and Eastern Boundary fault systems cut the arch (FIGURE 2). The paleoarch appears to have been a buttress to later fault deformation, as shown by the uplift of the southern and northern Attic fault blocks on the sides of the arch (FIGURE 9). The northwest-striking faults delineate the arch and are parallel to subparallel to one another and dip to the northeast and southwest (FIGURE 2, FIGURE 8, FIGURE 9, and FIGURE 10). Individual faults range from 1 to 5 km in length and are curvilinear with some showing oblique slip. Both planar and listric faults occur, and some of these faults have been re-activated as reverse faults during later compression and inversion. Typical secondary normal fault structures such as roll-over and drag folds and antithetic and synthetic faults are associated with these faults (Withjack et al., 1995). Many of these northwest-striking faults are deep-seated and define horsts and grabens on the arch. The Cretaceous to C-5 strata have been folded and faulted uniformly within this arch (FIGURE 7 and FIGURE 10). The arch is eroded, with almost all of C-4 and C-5 strata locally removed.

The origin of the paleoarch in Block I is related to the Merida arch and probably represents one of two foreland bulges that developed in front of a series of southwest-advancing thrust nappes related to the collision of the Caribbean and South America plates.

The West Flank or graben area of Block I is very different from the East Flank in both its structure and its stratigraphy. Structurally, the West Flank is downthrown 1-2 kms to the west along the Icotea /Attic fault system (FIGURE 8, FIGURE 11). This flank is characterized by three subparallel to parallel prominent northeast-striking fault zones that merge with the Icotea and Attic fault system (FIGURE 2 and FIGURE 3). These faults are linear, 12-15 kms long, and mainly have reverse motion with 50-100 m of displacement. At depth these faults merge with the Icotea/Attic fault zones to the east (FIGURE 5, FIGURE 6, and FIGURE 11). These structures are young and cut most of the older northwest-trending structures and the paleoarch, which are poorly imaged in the West Flank. This 3-D seismic interpretation of these faults and their trends is significantly different from the original mapping in the area.

The Eastern Boundary fault is a major north-northeast-striking fault system that occurs on the eastern side of Block I (FIGURE 1 and FIGURE 2). The fault zone consists of closely spaced, subparallel normal and reverse faults that bound the east side of the horst block and are subparallel to the Icotea fault (FIGURE 2, FIGURE 5, and FIGURE 6). This fault system merges with the Icotea fault north of Block I, forming a wedge-shaped basin between these two fault systems. This fault system continues to the north and south of Block I (FIGURE 2); in Block I, it is 16-20 km long with total vertical displacements up to 1 km. The main fault trace is listric and dips to the east-southeast with prominent roll-over fold locally dipping into this fault (FIGURE 5 and FIGURE 6). Locally, the fault has been inverted, re-activating the listric fault into a reverse fault, with the Miocene strata being folded and displaced above this fault. The main folds associated with this fault system are to the east of Block I in an adjoining concession. An asymmetric half graben, containing a clastic wedge, occurs in the extreme northeastern part of Block I (FIGURE 5 and FIGURE 6) along the Eastern Boundary fault. This clastic wedge thickens in the C-1 to C-4 sands interval to the east-northeast onto the adjacent area. In addition, it onlaps the arch in the south-central part of Block I and is not present in the south-southwest part of the block. The top of the wedge is characterized by an angular unconformity and the base by stratigraphic onlap.

The Tertiary tectonics of Block I are summarized by (1) early Eocene compression, forming reverse faults on both sides of the horst block, followed by uplift and erosion; (2) early Eocene extension and transpression along the side of a Jurassic graben system with C-7 to C-5 sands deposition; (3) compression, inversion, and block rotation in the middle to late Eocene with development of the paleoarch, folding, faulting, uplift, and local erosion followed by (4) stratal onlap and listric faulting over older structures during C-4 to C-1 sands deposition; (5) inversion during the late Eocene, with uplift and erosion; and, (6) following the early Miocene-related Andean orogeny, deposition was followed by inversion with re-activation of some of the older Eocene normal faults into reverse and thrust faults such as occurred along the Attic and Eastern Boundary faults (FIGURE 5 and FIGURE 6). Sedimentation occurred throughout the Eocene and was strongly influenced by tectonism, as evidenced by the development of paleoarch and listric faults along with the deposition of clastic wedges and stratal onlap surfaces.

SEISMIC STRATIGRAPHY AND SEDIMENTOLOGY

The stratigraphic section of the East Flank of Block I includes, from oldest to youngest: pre-Triassic basement rocks; the Jurassic graben La Quinta Formation; the Cretaceous Rio Negro, Cogollo Group, La Luna, Colon, and Mito Juan formations; the Paleocene Guasare Formation; the Eocene Misoa Formation; the Miocene La Rosa, Lagunillas, and La Puerta formations; and the Quaternary El Milagro Formation (FIGURE 4). Only the lower part of the Eocene Misoa Formation is preserved in Block I; it includes the C-1 to C-7 sands (FIGURE 5 and FIGURE 12). Most of the Eocene B sands and all of the Pauji were either eroded or not deposited in this area. The section on the West Flank is thicker than that on the East Flank and, in addition, contains the B-8 and B-9 sands.

The Block I Eocene section is bounded by two major bracketing unconformities, the E1 (25 Ma) and E2 (54 Ma) (Maraven Exploration Regional Study Team, 1995, personal communication). The Eocene stratigraphic section contains two main depositional sequences, the C-1 to C-4 sands and the C-5 to C-7 sands that are separated by an onlap surface at the top of C-5 (FIGURE 5). At least seven disconformities occur within the Eocene section and allow the section to be subdivided into eight sequences in the horst block (FIGURE 5 and FIGURE 12).

The E1 unconformity spans 16-50 Ma and is considered to be 25 Ma (late Oligocene) in this area. It is an angular unconformity and separates the Oligocene to Miocene basal Santa Barbara member (16-25 Ma) from the various Eocene Misoa C sands (45-54 Ma). This unconformity is very distinctive on seismic profiles and truncates all underlying strata (mainly C-3 to C-5 sands). The upper Eocene depositional sequence (C-1 to C-4 sands) is a wedge-shaped section that has stratal onlap onto the older C-5 sand and is shale prone. This sequence thickens to the northeast and is not found in the south-southwestern part of the East Flank. The lower Eocene depositional sequence contains C-5 to C-7 sands, which are subparallel to one another. This section has a relatively uniform thickness and is sand-prone across the East Flank of Block I. The lower E2 unconformity (54 Ma) occurs at the top of the Guasare Formation and is a major disconformity in the area. Incised valleys filled with C-7 sands are locally cut into this surface (FIGURE 12).

Several attempts at establishing a sequence stratigraphic framework for the Maracaibo Basin (Perdomo and Bot, 1986; Marais-Gilchrist and Higgs, 1993; Pestman et al., 1994; other unpublished studies by BP and TOTAL, among others) have been made. Our study is built on the recognition of major unconformities and seismic markers that have been age-dated by well control and paleontology by Maraven and S. A. Consultores CSC (W. Wornardt, 1995, personal communications) during the past 40 years (FIGURE 4, FIGURE 5, and FIGURE 12). There was no attempt to correlate the unconformities and sequence boundaries with the global sea level curves (Haq et al., 1988) as was done by Marais-Gilchrist and Higgs (1993). Instead, our study tried to integrate the local stratigraphic nomenclature of the Misoa C sand with third- and fourth-order seismic events, namely, the recognition of sequence boundaries, marine-flooding surfaces, and systems tracts at a seismic level and parasequences at a log-correlation level. We have found that several of the major unconformities are time-transgressive in this area and span long periods of time. For example, the E1 unconformity represents a possible 39 Ma erosional marker and/or nondepositional interval (from 16 to 45 Ma). In different parts of the Maracaibo Basin, the age relations of unconformities vary (Pestman et al., 1994), especially where they cut and merge with one another.

The C sands of Block I are up to 760 m thick and subdivided into eight sequences within the horst block (FIGURE 5 and FIGURE 12). They consist of transgressive, highstand, and lowstand systems tracts deposited in coastal and shallow-marine environments. Overall, the C sands in Block I form a retrogradational clastic sequence that is sand prone at the base, becoming more shale prone upwards (FIGURE 12 and FIGURE 13). This back-stepping pattern is attributed to rapid subsidence and net southwestward basin transgression during deposition of the C sands. Afterwards, the B sands were deposited as a progradational outbuilding sequence and are locally preserved in the West Flank. Facies and formation names change dramatically from southwest to northeast. The Misoa Formation varies in facies characteristics in both space and time. The paralic and shallow-marine Misoa Formation is transitional into the fluvial Mirador in the southwest, and may be laterally equivalent with the deep-marine late Paleocene to early Eocene Trujillo Formation and middle(?) to late Eocene Pauji to the northeast (FIGURE 13). Block I is located in the Misoa part of these facies associations (FIGURE 13 and FIGURE 14). Regionally, structure controlled sedimentation in the Maracaibo Basin. The ancestral Orinoco River was forced to cross several paleoarches that formed as foreland bulges subparallel to series of southwest-directed thrust nappes.

The Eocene section on the East Flank horst contains nine sequence boundaries (SB) and eight marine-flooding surfaces (MFS) that define eight sequences (FIGURE 5 and FIGURE 12). Sequence boundaries occur at the top of Eocene unconformity E1 (SB1), within lower C-2 sand (SB2), middle C-3 sand (SB3), within the C-4 sand (SB4 and SB5), middle C-5 sand (SB6), middle C-6 sand (SB7), lower C-6 sand (SB8), and at the top of Guasare unconformity E2 (SB9). Each of these disconformities or unconformities has a corresponding maximum marine-flooding surface. The Eocene sections east of the Eastern Boundary fault and west of the Icotea fault system have not been correlated in this study and are apparently different. The transgressive systems tracts make up about 70-80% of the C sands and are sandstone prone at the base (C-7 and C-6 sands), becoming more shale prone in the upper C sands (C-2 and C-3 sands). C-4 and C-5 sands contain nearly equal amounts of shale and sandstone (FIGURE 12). The transgressive systems tracts consist of stacked marine channels and bars arranged into thinning- and fining-upward or blocky intervals (FIGURE 12). There are two types: (1) thin basal transgressive sheet sands that are 10-30 m thick, and (2) thicker transgressive retrogradational intervals 100-200 m thick (FIGURE 12, FIGURE 14, and FIGURE 15). Both types of transgressive systems tracts are commonly overlain by thinner highstand systems tracts. The base of each transgressive systems tract is characterized by a sequence boundary and its top by a marine-flooding surface. Each systems tract consists of an erosional channel or channels characterized by bell-shaped log patterns that become more shaly upward. The seismic facies of the transgressive systems tract consists of discontinuous reflections of variable amplitude and are best viewed in horizon slices. The impedance contrast between the relatively thick sandstone intervals and the underlying and overlying shales usually produces several reflections. Downlapping seismic events occur locally at the lower sequence boundary, with parallel to subparallel reflections occurring at the upper marine-flooding surface. Horizon- and flattened time-slice maps show north-trending linear amplitude events interpreted to be channels and bar complexes (FIGURE 15). Net sand and oil accumulation maps mirror these amplitude trends. The transgressive systems tracts are the best reservoirs in the field area, with porosities in the 18-25% range and permeabilities up to several hundred md (Muñoz et al., 1994).

Highstand systems tracts comprise about 10-15 percent of the C sands and are mainly shale prone (FIGURE 12). Individual highstand cycles are 10-30 m thick and characterized at the base by a marine-flooding surface and at the top by a sequence boundary. The base of this systems tract is predominantly shale and is transitional upward into units with alternating sand and shale. On logs, this system tract is characterized by a shale, which forms seals over the transgressive systems tracts in these retrograding systems. At least seven highstand systems tracts are recognized in the horst block of Block I. Seismically, the highstand systems tracts occur as both continuous and discontinuous, subparallel reflections of variable amplitude. No distinctive toplap or baselap patterns were visible on seismic studies in this facies, probably due to the factor that the seismic resolution was below minimum thickness values of this facies for these depths. This is considered a nonreservoir facies in this area.

Lowstand systems tracts make up 10-15% of the Misoa Formation in the block. Both incised valley fills and lowstand wedges are recognized (FIGURE 12). The incised valley fills are found at the top of the Guasare, where the lower C-7 sands infill erosional channels. The incised valleys are less than 100 m thick, trend north-northeast, contain the coarsest sands in the system, and generally contain water in the field area. They have a blocky log signature, and core studies show tidal bundles characteristic of subtidal channels (well VLA-186). They are overlain by a transgressive systems tract in this area. At least two, and possibly three, lowstand wedges (above SB4 and SB6) occur in Block I and are best developed in the northern part of the block (FIGURE 12). They are 30-60 m thick and are characterized by thickening- and coarsening-upward log patterns of mixed sandstone and shale. The upper lowstand wedge (above SB4) onlaps and pinches out against the paleoarch. On seismic profile, the incised valleys can be locally seen in cross section, where they are enhanced by structure. Both the lowstand wedges and the incised valleys downlap sequence boundaries. Both facies consist of discontinuous reflections of variable amplitude. These two subfacies are locally productive in Block I, with porosities in the 12-20% range and less than 100 md.

In Block I, the main reservoirs are transgressive deposits of the Misoa C sands and the basal Santa Barbara member (FIGURE 4 and FIGURE 12). Reservoir trends and facies associations were determined using logs, cores, and amplitude and flattened horizon time-slice maps (FIGURE 14 and FIGURE 15). The composite geologic and depositional model (FIGURE 14) for the Eocene Misoa C sands recognizes north-trending marine channels and bar systems deposited mainly subtidally in front of a back stepping tidally influenced delta system (FIGURE 14 and FIGURE 15). Water injection, production, petrophysical, and geochemical data shows similar north-south trends (Muñoz et al., 1995) that match the seismic amplitude patterns in this area. Recently found microfossils in marine-flooding surfaces in the lower C-4 sands suggest inner neritic depths for Block I (W. Wornardt, 1995, personal communications). Modern examples of tidally-influenced transgressive deltas that may be similar to the Misoa C sand-rich system include the subtidal part of the Colorado River delta in Mexico, the Mahakam delta in Indonesia, and the Fly delta in Papua New Guinea (Meckel, 1975; Galloway and Hobday, 1983). In the Eocene B sands in the onshore Lagunillas field, Maguregui and Tyler (1991) also documented a tidal deltaic paleoenvironment setting (FIGURE 1).

EASTERN BOUNDARY FAULT PROSPECT

Using 3-D seismic data, the Eastern Boundary fault was remapped and a new structural model was developed. Older 2-D data coverage was inadequate in this area, and no wells had been drilled in this area of Maraven's acreage during the past 36 years. The use of 3-D seismic data allowed this complex structure to be imaged for the first time and the locations of previous wells to be better evaluated.

New Concept From 3-D Seismic Data

The Eastern Boundary fault is interpreted to be a narrow, complex, north-northeast striking fault zone within a syncline. It is more than 10 km long and 0.5-1 km wide (FIGURE 16 and FIGURE 17). Originally, the fault formed as an east-dipping listric fault; it was later inverted, forming a small and distinctive anticline cut by one or more northeast-striking antithetic faults that dip to the west and by several northwest-northeast-striking normal and reverse cross faults. The main structure can be subdivided in most places into three distinctive parts: (1) roll-over fold, (2) upthrown antithetic, tilted-fault block, and (3) downthrown, antithetic fault block (FIGURE 17 and FIGURE 18). The inversion structure clearly formed post-Eocene, as seen seismically by the folding and uplift of the Eocene unconformity. Much of the fault zone is outlined by distinctive higher-amplitude seismic events, which allowed this trend to be accurately mapped and potential reservoirs to be calibrated (Figures FIGURE 19 and FIGURE 20). All the previous wells in this area were drilled into the upthrown and/or the downthrown parts of the main antithetic fault(s) that dipped into the Eastern Boundary fault (FIGURE 18). These wells had mixed results based on the close proximity of reservoirs to sealing faults. Based on the new mapping, a roll-over fold prospect with four-way closure was identified in a fault block, approximately 0.8 km long by 0.5 km wide, that was bounded by faults on all sides (Figures 16-20).

Results

Three wells were drilled during late 1994 and early 1995 (VLA1131; its twin, VLA1147; and VLA1159) to test the prospect. A 1000-m-thick hydrocarbon-bearing zone, including the Miocene Lagunillas Formation, the Santa Barbara member of the La Rosa Formation, and the Eocene Misoa C-1 to C-6 sands (FIGURE 4), was encountered with original pressures in the fault block. Gas and condensate were tested in the C-1 to C-5 levels and are believed to be responsible for the higher-amplitude seismic events within this fault zone. Oil was found in the Miocene strata and the Misoa C-5 and C-6 sands. Initial flow rates were 1400 BOPD from the C-6 sand, 200-300 BOPD from the C-5 sand, and 8 MCFGD from the C-3 sand. A check-shot survey and synthetic seismogram were used to calibrate the 3-D seismic data, and better identify the reservoirs and the amplitude events (FIGURE 20). A major sequence boundary or angular discordance recognized on the 3-D seismic data at the top of the C-5 sand (FIGURE 19) may allow the Eocene reservoirs here to be developed as two separate reservoir units (C-6/C-5 oil-bearing unit and C-1 to C-4 gas-bearing units).

Based on the success of these wells in this synclinal area, several new well localities and prospects have been identified. In addition, further testing of the existing reservoirs and mapping of the multiple reservoir intervals of this area are planned to optimize field development.

VLA1035 HORIZONTAL WELL IN THE ATTIC

This development well was drilled in 1992 and required a horizontal section to drain the reservoirs from an Attic fault block, which have been producing oil for more than 40 years (FIGURE 21 and FIGURE 22). The percentage of water in the produced oil zone in this block had increased from 20% in 1960 to over 85% in 1991. Water-coning problems in the field had rendered any additional vertical wells uneconomical because of the steep dip of the fault block and the strong water drive. The remedy was to drill a horizontal well in the upper zone of the producing structure, targeting a thick continuous sand at the top of the oil column. The 3-D seismic data coverage played an important role in selecting the well location by documenting existing faults and their positions, stratigraphic and amplitude trends, and terminations and pinchouts of strata. In addition, the use of closely spaced (40 m) inlines and random well-tie lines allowed accurate mapping of the dip and strike of the C-7 reservoir, which was critical for locating the drainhole within the productive interval. The strike of the reservoir horizon was parallel to the sealing Icotea fault and dipping steeply to the east. The structure consisted of an upthrown reverse fault block, or "pop-up" structure, bounded on the west by the Icotea fault and to the east by the Attic reverse fault (FIGURE 21). The Attic fault block here is only 0.6 km wide and less than one km long. The horizontal well was drilled parallel to the strike of the reservoir in an updip position above the existing oil-water contact. The well trace followed a high-amplitude reflection calibrated to be part of the upper C-7 reservoir target that was interpreted to be a channel, based on sedimentologic studies (FIGURE 23).

Results

The VLA1035 well was Maraven's first horizontal well in Lake Maracaibo and successfully utilized a new drilling technique combined with 3-D seismic data to better develop existing field areas (Belloso et al., 1994). More than 300 m of horizontal pay was drilled in the well in continuous channel sand complex estimated to have a real vertical thickness of only 12 m. The well originally tested more than 2400 BOPD, has produced more than 800,000 BO between 1992 to 1995, and is still producing more than 700 BOPD.

Based on the success of this well, four more horizontal wells were drilled in the northern Attic region of Block I, mainly as re-entry localities (FIGURE 22). In addition, several new horizontal and highly deviated wells are planned for areas that require maximum penetration of highly dipping strata in the Attic fault zone in the near future.

CONCLUSIONS

(1) Further development of older producing fields, such as Maraven's Block I, can be greatly improved by using 3-D seismic data combined with all other available data. Remapping of stratigraphic tops, horizons, and faults, combined with production data, allows the evaluation of earlier wells and the accurate location of any future wells and/or injectors. In particular, the precise location of fault and stratigraphic terminations can be accurately determined and mapped.

(2) A new, more complete structural model has reduced the necessity for using over half of the original faults recognized in the field. Complex faults, such as the Attic, Icotea, and Eastern Boundary faults were found to consist of several fault traces and to be locally compartmentalized. Stratigraphic tops and horizons were combined with sequence boundaries, marine-flooding surfaces, and depositional sequences to better characterize reservoir trends and facies in the field. Predictive depositional models were made utilizing all available data. The interpretation of seismic horizons, various derivations, and fault polygon maps have led to the complete revision of the structural and stratigraphic models for Block I.

(3) Using 3-D seismic and log displays in the form of cross sections, fence diagrams, seismic cube and chair displays, isometric projections, and time and horizon slices greatly improved confidence in drilling any prospect or infill locality. The combined graphic representation of seismic, log, and map data allows greater visualization of prospects and concepts to be presented to management.

(4) 3-D seismic data were used in Block I to better locate wells and to develop new exploration and production concepts as shown in the two successful wells drilled: the VLA1131 discovery along the Eastern Boundary fault and the VLA1035 horizontal well in the Attic fault zone. Both wells tested concepts for developing new areas in an old producing field and led to drilling of other wells using these new concepts.

ADDENDUM

In the year following the writing of this chapter, Maraven tested the three wells described in the VLA1131 Discovery section. The wells (VLA1131, VLA1147, and VLA1159) initial production totalled 1392 BOPD; VLA1131 tested 264 BOPD, VLA1147 produced 825 BOPD, and VLA1159 tested 305 BOPD from single horizons. Each of these wells will develop other horizons through their life to better exploit and develop the thick potential stratigraphic section present in each well.

Acknowledgments

We thank the many people who have contributed to this project, particularly the Maraven Production of Segregación Lagomar Group; Maraven Exploration Regional Study Team; Maraven/Amoco Cretaceous project, S.A. Consultores CSC (Muñoz et al, 1994); Maraven Sedimentology (E. Sampson and F. Chacartegui) and Geochemistry groups (L. Mompart); log correlation and petrophysical mapping by S.A. Consultores CSC (P. Jam, J. Delgado, and A. J. Guerrero); initial sequence stratigraphy interpretations by R. M. Mitchum and paleontologic studies by W. W. Wornardt. P. Bartok, S. K. Ghos, P. J. Pestman, and P. Weimer reviewed various drafts of this manuscript and earlier abstracts. Maraven Exploration Regional Study Team kindly provided the age and stratigraphic relationships in the Maracaibo Basin and discussed their original work with us. Some of the figures used in this paper were drafted by C. A. M. Edo and S.A. Consultores CSC. Maraven S. A. supported this project and kindly gave permission that it be published.

REFERENCES CITED

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FIGURE 1. Index map showing Lake Maracaibo. (A) Map of major onshore fields, trace of the Icotea and Eastern Boundary faults, location of Lagocen regional seismic line 15, Jurassic graben outline (Bartok et al., 1981), and position of Block I. (B) Sketch of west-east Lagocen 15 seismic line, highlighting the Jurassic basin and Icotea and Eastern Boundary fault-bounded horst of Block I, and major unconformities (U).

FIGURE 2. Simplified structure map at the top of the Guasare Formation for Block I, modified from the Guasare first derivative or dip map shown in FIGURE 3. Major faults; the location of the paleoarch; Block I and seismic survey outlines; the horst and graben; and West and East Flank basins are shown. The position of inline 40 and crossline 417 are indicated. The location of Horizontal Well and the Eastern Boundary Fault Prospect areas are highlighted with a box.

FIGURE 3. Dip or first derivative map on the top of the Guasare Formation for the entire block, showing major fault (F) patterns, position of the paleoarch, West and East flanks, and inline and crossline numbers.

FIGURE 4. Block I stratigraphic section, highlighting ages, rock units (formations), unconformities, and major seismic markers.

FIGURE 5. Geologic sketch of the horst area of Block I, showing the unconformities, major structures, sequence boundaries, various C sand members of the Misoa Formation, and the major stratigraphic relations for the East Flank of Block I. Timing of structural features from oldest to youngest is indicated with (1)-(4). See FIGURE 6 for seismic line interpretation.

FIGURE 6. West-east inline 40 across the East Flank of Block I, highlighting the major structures, unconformities, and stratigraphic relationships. See FIGURE 2 for location of inline; FIGURE 4 for seismic markers; and FIGURE 5 for schematic interpretation of faults, the various C sands, and sequence boundary localities. Seismic markers include the Lower Cretaceous (LK) unconformity, Upper Cretaceous Socuy member, top of the Paleocene Guasare Formation, and top of Eocene Unconformity (see FIGURE 4).

FIGURE 7. Chair display of the complete 3-D seismic survey, looking northwest at the East Flank of Block I. The Icotea fault, the southern and northern Attic blocks, northwest-striking faults that delineate the paleoarch, inline and crossline localities, top of Guasare and Eocene unconformity seismic markers, and depth in time (ms) (see slice) are indicated.

FIGURE 8. A 3-D isometric projection on the top of the Guasare Formation in the West Flank, looking northeast and highlighting the Icotea and the subparallel reverse faults of the West Flank that merge with the Icotea fault forming triangle- to wedge-shaped fault blocks. The southeast corner of the West Flank adjacent to the Icotea fault is down-dropped about 1/2-1 km to the west, whereas the northeast corner adjacent to the Icotea fault has a displacement of almost 2 km.

FIGURE 9. A 3-D isometric projection on the top of the Guasare Formation in the East Flank, looking northwest to highlight the Icotea and Attic faults. The northern and southern upthrown Attic blocks are bounded by reverse faults that dip to the west and are separated by a saddle in the middle. The saddle coincides with the paleoarch, which occurs near the south-center of the East Flank and is delineated by northwest-striking normal faults that dip both to the northeast and southwest. The two Attic fault blocks may have formed due to shortening of thicker section on sides of the paleoarch during later fault deformation.

FIGURE 10. North-south crossline 417 across the East Flank, flattened on the top of Eocene Unconformity, and highlighting the paleoarch and its northwest-striking faults (F). See FIGURE 2 for location of crossline. Seismic markers include the Lower Cretaceous (LK) unconformity, Upper Cretaceous Socuy member, top of the Paleocene Guasare Formation, and top of the Eocene Unconformity (see FIGURE 4). North-to-south onlap (arrows) can be seen above the Guasare marker, starting just above the 500 ms depth that is flattened on the Eocene unconformity.

FIGURE 11. Chair display of the 3-D seismic survey, looking northeast at the West Flank of Block I. The Icotea fault strikes northeast across the chair display and is characterized by a nearly vertical trace that is offset and displaced by the Attic fault (best seen in the upper central part of the chair display). Northeast-striking antithetic reverse faults are subparallel to and dip into the Attic/Icotea faults. The Eocene section is 1-1/2 times thicker on the West Flank than on the East Flank. Attic blocks subparallel the Icotea fault to the east. Inline and crossline localities; top of Cretaceous, Guasare, and Eocene unconformity seismic markers; vertical scale in time (ms) (see slice 4000, 3188, 2748) are indicated.

FIGURE 12. Type log and sequence stratigraphic diagram for the Eocene Misoa C sands in the East Flank of Block I, showing tops, sequence boundaries, marine-flooding surfaces, and systems tracts. This stratigraphic log is a composite log for the area east of the Eastern Boundary fault.

FIGURE 13. Stratigraphic model for the Eocene strata of the Maracaibo Basin. (A) Simplified early to middle Eocene paleogeographic map, showing southwest to northeast transition from fluvial to shallow-marine to deep-marine facies relations; and (B) southwest to northeast stratigraphic cross section, highlighting the transition from the Mirador to the Misoa to the Trujillo formations and from the nonmarine to the deep-marine facies relations, the various B and C sands of the Misoa Formation, and the retrograde (back-stepping) of the C sands vs. progradation (out-building) stacking patterns of the B sands.

FIGURE 14. Depositional model for the Misoa and Mirador formations at C-5 to C-7 time. A retrograde back-stepping pattern of sand-prone sedimentation occurred during lower Misoa deposition. Shallow-marine facies (Misoa) were deposited and are transitional from nonmarine units (Mirador) to the southwest to deep-marine facies (Trujillo) to the northeast. Depositional trends are aligned northwest, north, and northeast and follow the original confining shape of the northeast-trending basin.

FIGURE 15. Flattened horizon-time slice and sketch at the C-5U horizon level. (A) Horizon-time slice taken at 260 ms above the top of Guasare. The north-trending linear amplitudes are cut by major northwest-striking faults (white fault traces). (B) Sketch of the north-trending amplitude patterns interpreted from these amplitudes and other core and log data to be subtidal marine channels. Fault abbreviations: D = down; U = up.

FIGURE 16. Simplified structure map of the Eastern Boundary fault, showing wells, prospects, trends of seismic amplitude events, and major fault patterns at the C-6, C-5, and C-4 levels. The positions of inline 28 and crossline 605 are shown in FIGURE 18, FIGURE 19, and FIGURE 20.

FIGURE 17. Composite structural model for the Eastern Boundary fault based on sketches made from west-east seismic lines, highlighting the compartmentalization of the narrow structure into (1) a roll-over fold, (2) upper antithetic, tilted-fault block, and (3) a lower antithetic, tilted-fault block. Horizons and the relative positions of key wells are indicated (see FIGURE 16).

FIGURE 18. Fence diagram of the VLA1131 well, showing the structural closure, faults, and unconformity at the top of the C-5 horizon. West-east inline 28 and north-south crossline 605 make up the two sides of the fence diagram. See FIGURE 16 for location of seismic lines. The high-amplitude seismic events outline the C-4 to C-6 reservoirs.

FIGURE 19. North-south seismic crossline 605, showing the structure and amplitude patterns for part of the Eastern Boundary fault. Top of C-5 unconformity, major faults, and stratigraphic tops are indicated. Note the reversal of dips below the unconformity.

FIGURE 20. Synthetic seismogram, gamma ray log, and west-east inline 28, showing the structure and log character for the VLA1131 discovery well and the VLA1147 twin well. This display shows, from left to right: sonic log, reflectivity, synthetic seismogram, seismic reference trace (trace at well location), and an interpreted section of inline 28 from the 3-D seismic survey with gamma ray log overlain at the well location. Other abbreviations: BLR = basal La Rosa Formation, ER-EO = Eocene Unconformity, and CDP = common depth point.

FIGURE 21. Index map for the VLA1035 well in the northern Attic fault block. Contours and structures are on the top of the C-7 sand. Key wells and seismic lines, position of the chair display, and isometric projection area for this part of the northern Attic fault zone are indicated.

FIGURE 22. Isometric projection of the northern Attic fault block at the top of the Guasare Formation, showing the location of the VLA1035 well and four other horizontal wells drilled between 1992 and 1995. The success of the VLA1035 well led to the drilling of the VLA408, 459, 780, and 807 re-entries as horizontal or highly deviated wells in this fault block.

FIGURE 23. Chair display of part of the northern Attic block and the VLA1035 well, highlighting the amplitude trends at the top of the C-7/lower C-6 horizon. These amplitude patterns are interpreted to be a map view of north-trending channels as viewed as a chair-time slice and their discontinuous lenticular channel cross sections in an west-east inline direction. See FIGURE 21 for location of chair display.