uAbstract

uFigures

uIntroduction

uMethodology

uElectrical Logs

uSeismic Attributes Analysis

uConclusions

uAbstract

uFigures

uIntroduction

uMethodology

uElectrical Logs

uSeismic Attributes Analysis

uConclusions

uAbstract

uFigures

uIntroduction

uMethodology

uElectrical Logs

uSeismic Attributes Analysis

uConclusions

uAbstract

uFigures

uIntroduction

uMethodology

uElectrical Logs

uSeismic Attributes Analysis

uConclusions

uAbstract

uFigures

uIntroduction

uMethodology

uElectrical Logs

uSeismic Attributes Analysis

uConclusions

Introduction 

The Petropiar field is part of the Ayacucho Area, located in the Orinoco Oil Belt (Figure 1). The area is structurally simple, with an igneous /metamorphic basement of Precambrian rocks, dipping slightly to the North that has been reactivated by several faults with direction East-West and Northwest-Southeast. The sedimentary column, mainly sand and shale intercalations, presents a heavy oil reservoir of Lower to Middle Miocene, deposited under fluvial and fluvial-deltaic environments. The stratigraphic seal is controlled by faulting. 

The four stratigraphic surfaces comprise a stratigraphic column of great economic importance. These are identified as AMV-125 (Lowstand System Tract), AMV-145 (Transgressive System Tract) Early Miocene, AMV-200 (Sequence Boundary) and AMV-255 (Lowstand System Tract) Middle Miocene. 

Thirty-five wells were selected within the block, and these data used to generate several crossplots, that allowed the interpretation of the behavior of sand – shales in a seismic to well correlation. 

The 3D seismic volume (387 km²) , is a stack version, 8 bit format, zero phase and dominant frequency between (35 - 40) Hz with a vertical resolution already 40´. Both the seismic cube and the four surfaces were applied around fifteen seismic attributes, basic attributes and combinations of these, using visualization techniques. 

Methodology 

The methodology is summarized in the interpretation of lithology logs (crossplots), a simple and multiple combination seismic attributes for volumes and surfaces using visualization techniques, as well as net sand facies maps and sedimentological information. 

The objective was to understand the geological model through the different changes of the seismic signal, to search the relationship between lithology, sand bodies, deposition event, environment type and petrophysical parameters. 

The crossplots help to understand the relationship between the seismic response and lithology (sand - clay). Net sand facies maps, used as stratigraphic model of chronostratigraphic Tops AMV-125, AMV-145, AMV-200 and AMV-255, were dated through previous Micropaleontological and Palynological studies. Two main types of environments were distinguished: fluvial and deltaic, defined as boundaries of sequences of third and fourth orders, within a general third order model. Each of these surfaces is represented in the wells within a classification of sedimentary facies: lithological, biodepositional and depositional log. 

Electrical Logs 

Thirty-five wells were used, 19 vertical wells with basic logs, and 16 horizontal wells with interpreted logs. VSH (Shale Volume) was calculated as the product of the correction to GR and Neutron-Density to obtain the percentage of clay and separation to clean sand. This data was taken as a color scale in the crossplots: Depth vs. Shale Volume, Density vs. Sonic impedance, Velocity vs. depth, P-wave impedance vs. Neutron, Neutron vs. S-wave impedance vs. GR, Speed vs. Density. 

The GR cut for clean sand was 28 ° API and for the shales 130º API. Generally, the greater net sand thicknesses are preferable located to the northern of the block of Petropiar, while the dirty sands are located to the south of the block. For the top-125 AMV and AMV-255 the sand percentages are higher than those of the shale, the top-200 AMV the sand and shale relations is nearly equal, mostly sand mixed, while for the top-145 AMV sands are mixed. 

For the DT vs. RHOB, the difference is marked by the density values (2.3 g / cc shale and 2.1 g / cc clan sand), due to the fact that the velocity values for clean sand, mixed sand and shales are very similar. (Figure 2) Crossplot NPHI vs. impedance, the total porosity allows separate three lithologies, 50% to 32% for shales and sands, values are maintained throughout the Ayacucho block.  For the P wave impedance, larger sample difference between the clean sands and shales (Figure 3) are observed, a result that is not so obvious in the impedance and S wave in DT, which results in a poor seismic and well match. 

Seismic Attributes Analysis 

Fifteen seismic attributes were used, and 8 attributes volume: instantaneous amplitude, instantaneous amplitude / √ frequency (Figure 4), peak frequency onlap, frequency trough onlap, maximum phase, phase + π, trough phase, phase zero (P / T) 7 and surface attributes: attribute x thickness, maximum positive attribute, attribute positive minimum, maximum negative attribute, attribute absolute maximum, RMS, average absolute attribute, mainly attributes that identify stratigraphic events 

For each attribute we used visualization techniques in order to reach the best correlation with net sand facies maps, taking highlight the values of greater amplitude. (Figure 5) 

For surface attributes a window was taken (+ / - 5) ms, where positive values (red) gives clean sand thickness and negative values (blue) are clays thickness. 

In general, a good coupling is showed between instantaneous amplitude, phase (phase + π), phase (trough) and multiple attribute (amplitude snapshot / √ frequency) attributes with net sand maps. However, the multiple best attribute resulted with the top AMV- 125 (Figure 6), because this attribute separates lithology of fine- thick grained in siliciclastic environments, clean sand with high percentages of quartz grains reworked in fluvial channel, deposits in flood plains in a Lowstand valley sequence. 

Top AMV- 145 (TST), the sand trend is similar to the distribution of the instantaneous amplitude map but is not exactly the same, this is because the sand bodies are thinner, below the resolution seismic and its development in the coastal plain and flood plain deposits are Buddy channel, overbank and splay system where the lithology is clay. 

For the AMV-200 (SB), a relationship between the sedimentology model and attribute maps was not able to be obtained, which is higher due to the presence of fine grained sand, which developed along the coastal plain, and coast and bay platform; basically overbank deposits. While for the AMV-255 which is another big net sand thickness Lowstand, the relationship is not clear as for the top AMV-125, because the higher sand percentage is mixed with clean sand with high of feldspar content, less reworked, thus the presence of fine grains that are deposited along the flood plain, coastal plain and bay shore deposits of mud channels, overbank. (Figure 7) 

Conclusions 

The purpose of this study was to characterize the continuity of the reservoir areas through various combinations of seismic attributes to obtain a geological response directly and that would depend on the quality of the seismic, environment type, sedimentary deposited and lithology. 

Although through the relationship of crossplot showed little difference in velocity between the three lithologies (clean sand, mixed sand and shale), and little association with this seismic-well, through the parameters of the total porosity, density and p-wave impedance, if it was possible to separate them. 

The seismic attribute depends on the seismic quality and could improve its response based on different combinations of attributes that were generated to achieve the best fit between the attribute and sedimentology model. 

As a result, it is concluded that the multiple attribute maps and instantaneous amplitude is more similar to the response of the facies maps, if we are in a fluvial depositional environment. For top-125 AMV the conditions clean sand fluvial channels under a realm of floodplain within a limit of sequence Lowstand System Tract, its continuity is clearer over multiple attribute map. For tops AMV AMV-145 and AMV-255 is less obvious to maintain the continuity of the proposed net sand map of the facie model, since the deposits are mostly splay system, and hybrid channels in overbank environments and coastal plain, bay coast. While for the top-200 AMV, a direct link between the attribute maps and facies model, was almost zero. There was a greater presence of fine-grained lithology as overbank deposits that developed along the coastal plain, bay coast and platform. 

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