Strike-Slip Model for the Jacinto and Paredón Fields of the Chiapas-Tabasco Region, South East Basin, Mexico*
J. Fernando González-Posadas1, Salatiel Avendaño-López1, and Jorge Molina2
Search and Discovery Article #20028 (2005)
Posted June 16, 2005
*Adapted from extended abstract, prepared by the authors for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004.
1PEMEX, Villahermosa, Tabasco ([email protected])
2Independent, Colombia, currently Geoproduction S.A. ([email protected])
The Jacinto and Paredón fields are located 46 km SW of the city of Villahermosa, Tabasco, Chiapas-Tabasco region, also known as the Akal-Reforma horst (Figure 1). The Jacinto field, discovered in 1984, produces oil, gas, and condensate (45° API) from the Jurassic Tithonian and Lower Cretaceous rocks. The original recoverable reserves were 474.8 MMBOE. The Paredón field was discovered in 1978 and produces oil (39° API) and gas from the Lower Cretaceous, Tithonian, and Jurassic Kimmeridgian rocks. The original recoverable reserves were 162.66 MMBOE.
Early structural interpretations in this area were based on 2D seismic data and resulted in compressional models, principally thrusting similar to those observed in the Chiapas Range; therefore, the Jacinto field was interpreted as an asymmetric anticline oriented NW-SE and separated from the Paredón field by a normal N-S-trending fault; a series of normal faults divide the block into multiple compartments (Figure 2A). The Paredón field was interpreted as an elongate anticline trending NW-SE and bound by a reverse fault along its NE limit, as well as a series of normal faults with variable orientations (Figure 2B).
The Tepeyil 3D survey acquired in 2001 (Figure 1) confirmed the presence of thrust faults in Jujo-Tecominoacán and Cárdenas; however, there are also essentially vertical faults in the Jacinto and Paredón fields that cannot be readily explained by thrust faulting. The new seismic survey has allowed for a revised interpretation and has led to the application of a strike-slip model. This model is very difficult to visualize with 2D seismic due to the fact that to determine its presence it is necessary to observe lateral offset. This offset can best be observed on time slices based on the 3D survey. As a result of the observation stated above, there have been very few authors proposing strike-slip faults in the region (Delgado-Argote and Carballido-Sanchez, 1990). The mentioned authors proposed a transpressive regime associated with a left-lateral strike-slip fault extending from Puerto Angel, Oaxaca, to Macuspana, Tabasco. In the Chiapas region, south of the study area, strike-slip faulting has been reported by Meneses-Rocha (1987), Carfantan (1981) and Velez-Scholvink (1990); and it has been related to the Motagua-Polochic-Jocotan fault system.
The study area has been subjected to several compressive events, products of the collision between the Chortis and North American plate associated with a NE displacement that is related with the Motagua-Polochic-Jocotan transform. The first collision is recorded in the Late Cretaceous, resulting in the first uplift of the SE of Mexico and consequent effects on the carbonate to clastic depositional systems. A second compresional event occurred during the Late Eocene to Early Oligocene (Figure 3A) creating a series of low relief structures and erosion. A third event, and the strongest of the three, occurred during the Middle Miocene to the Early Pliocene time (Figure 3B) (Oviedo-Pérez, 1996, in González-Posadas, 2003). This last event has been named “Chiapaneco” and corresponds to the principal compressive phase observed in the Chiapas Range. It caused the bulk of the structures associated with the oil fields of SE Mexico.
The oldest strata drilled in the Jacinto and Paredón fields are Kimmeridgian in age. The reservoirs are Lower Cretaceous, Tithonian, and Kimmeridgian in age. The Lower Cretaceous consists mainly of fractured and brecciated fine- to medium-crystalline dolomites (Figure 4A), chert, and scarce planktonic mudstone-wackstone. These sediments were deposited in an open marine setting. Multiple stages of fractures are present. Crackle breccias, which are formed of angular dolomite fragments, appear to be the result of tectonic processes, such as faulting and folding. Some of the most intensely faulted breccias contain mylonite between the clasts, and as such, little or no porosity remains. It is the less intensely faulted breccias that commonly contain the best vuggy and fracture porosity. The Tithonian strata are mainly fine- to medium-crystalline dolomites and a few planktonic wackestone beds. They are commonly unfractured, but some intervals apparently from a highly faulted zone are intensely fractured (Figure 4B). An important charasteristic of these rocks is their tight, dense appearance, and yet there is hydrocarbon production. The Paredón-54 well produced from the Tithonian strata 771 BO and 22.4 BCF per day. These high flow rates indicate a very permeable reservoir that its porosity and permeability are inferred to be product of intense tectonic brecciation (Longman 1995, 1996).
The Kimmeridgian consists of fractured dolomites. Multiple stages of fractures are present; the later fractures are cemented with a combination of dolomite and calcite. Bitumen is also present in the later-stage fractures. Intensely sheared rocks commonly contain mylonite between the dolomite clasts. Peloids and grain ghosts suggest the original rock was a packstone deposited in fairly shallow water in a restricted platform setting. The reservoir rocks have good porosity and permeability because of fractures and breccias resulting from a late-stage tectonism (Longman 1995).
Our interpretation of the Jacinto and Paredón fields leads us to believe that the observed deformation is due to strike-slip faulting. The fields show the presence of almost vertical faults that are lost at depth in reflectors possibly associated with autochthonous salt (Figure 5A). The fault blocks vary from normal to reverse along the trace of the faults. On time slices they show lateral offset (Figure 5B). According to Harding (1990) the characteristics listed above suggest strike-slip faulting; however, it is not possible to observe the presence of displacement in the basement due to the difficulty in establishing continuity beneath the autochthonous salt.
There are two preferred fault orientations, the first orientation consists of nearly vertical faults, with a N-S direction. These faults are Paleogene in age and are present in the Jacinto Field (Figure 5A) and along the southern portion of the Paredón field. In the latter location, they are associated with a salt intrusion. In this system, Fault-4 (Figure 6A and 6B) with an apparent left lateral strike-slip displacement is the principal fault which separates the Jacinto and Paredón fields. The other fault system is composed of nearly vertical faults that at depth tend to merge into blocks with the appearance of a positive flower structure. Their orientation is NW-SE and are Miocene in age. This fault system is observed in the Paredón field (Figure 5A). Fault-27 is the principal fault in these field, as evidenced by its large diplacement. It is a left-lateral strike-slip fault that divides the Paredón field into two major compartments (Figure 6B).
The strike-slip model for the Jacinto-Paredón area implies that a new focus may need to be applied to describe the reservoir rock. In accordance with Antonellini and Mollema (2000), the Triassic Sella Group dolomites from north Italy were deformed in a tectonic strike- slip regime; compression caused the formation of fractures and strike-slip faults. The mentioned authors estimated the petrophysical properties of the dolomites in relation to fault offsets. Small-offset faults (up to 30 mm) are characterized by en echelon arrays of fractures and breccia zones up to 10 mm wide; they form areas of high permeability (100-3000 md). Faults with 1-10 m offsets, characterized by a breccia zone (1-2 m wide) and associated with high fracture density, contain high-porosity (10%) breccia and represent areas of preferred fluid flow. Large-offset faults with offsets more than 10 m contain a wide zone of low-porosity (<1%) breccia and form potential permeability barriers. The areas adjacent to the intermediate and large offset faults have high permeability (100-3000 md) because of high fracture densities (Figure 7A).
The dolomites of the Sella Group provide a good analog for the dolomites observed in the Jacinto Paredón fields because both were subjected to strike-slip faulting and contain extensive brecciated dolomites, breccias that may have developed from deformation associated with strike-slip faulting. In the analog the fault with the greatest displacement measured 200m, and in our area they measured 700m; however, comparisons can still be made. The faults that have the greatest displacement are Fault-27 (700 m) and Fault-4 (400 m). According to the model, faults with displacement greater than 10 m (both of the faults named above) would constitute permeability barriers, and it is suggested that Fault-4 does just that. It separates the Jacinto and Paredón fields where their original oil water contacts implied the presence of a barrier. The Jacinto field had an oil/water contact at 6325m while in the Paredón field the contact was at 6090 m. The spill point by juxtaposition between the two fields is observed at 5750m. The oil column implies the presence of a permeability barrier. Furthermore, the oil of the two fields is different. Jacinto is a gas field, and Paredón is a oil field. This difference further supports the presence of a barrier.
As a consequence of what has stated previously, it is deduced that faults with long displacements (>10m) act as seals; there are several faults in the area that fit the model. However, it is also observed that close proximity to the faults results in high permeabilities (due to intense fracturing), as is demonstrated by the well production histories. In order to demonstrate the relationship between cumulative production and proximity to faults, a graph was prepared for the Paredón wells (Figure 7B). On the graph the cumulative oil equivalent was plotted against the distance from the major fault. In general, wells lying between 40 and 200 m from the faults have higher production, and the wells that cut the fault (or are very close to the fault) were nonproductive.
Based on the model described above, five locations have been proposed: Uayum-1 (Figure 6A), Izcalli-1, Kayeb-1, Cuauhtli-1, and Camale-1 (Figure 6B). Their objectives are fractured dolomites of the Lower Cretaceous, Tithonian and Kimmeridgian strata. The first three were proposed based on the strike-slip model. Only the Kayeb-1 was affected by a salt intrusion. They are all field extensions, and the expected fluid type is gas and condensate. Cuauhtli-1 forms part of the positive flower structure. It is also a field extension well, and the fluid type is expected to be extra light crude. Lastly, the Camale-1 location is proposed to test a block affected by both strike-slip faulting and salt intrusion. It is on an adjacent block to the Paredón field and is expected to produce super-light crudes.
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