Recognition
of Fault
Bend Folding, Detachment and Decapitation in Wells, Seismic, and Cores
from Norte Monagas, Eastern Venezuela*
By
Chatellier, J-Y.,1 Hernandez, P.,2 Porras C.,2 Olave, S.,2 and Rueda, M.2
Search and Discovery Article #40031 (2001)
1PDVSA Intevep Apdo 76343 Caracas Venezuela
2PDVSA Oriente, Estudios Integrados Pirital, Puerto La Cruz, Venezuela
**Adapted for online presentation from poster session presented at the AAPG Convention, Denver, CO, June, 2001.
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Our study focused on Santa Barbara Field with integration of our previous studies of the Carito and Furrial fields (Figures 1.1 and 1.3). Various 3D seismic surveys and more than 300 wells on an area of 45 by 12km correspond to the major producing area in Venezuela with in place reserves of 26 MMMbbls and 50 TCF. Our integration was helped by observations and studies from cores, dipmeters, geochemistry, and pressure surveys.
Fault
Bend folding (Figure 1.2) recognized on various structural sections has been
corroborated by outstanding observations from cores. Regularly spaced flats and
ramps of small scale
fault
bend folds are characterized by slickensides,
pressure solution features and quartz cement precipitation.
The use of 3-D statistics enabled us to recognize highly prominent and laterally extensive detachment planes which have been corroborated by our stratigraphers and our reservoir engineers. Abnormal but highly characteristic RFT patterns have been successfully integrated in our study and confirm the small-scale observations made in cores.
One
school book example of a large scale decapitation came out of our fault
analysis
that included generation of
fault
planes,
fault
throw maps, birdseye maps and
integration with dipmeter and seismic. Thus along the same low angle
fault
plane, perfectly tied by fifteen wells on an area of 5x5 km, apparent throws
gradually change from a repetition of 510 ft to missing sections of more than
3000 ft. Folding occurred before the decapitation phase.
The 3-D seismic confirmed the existence of detachment and decapitation postdating a first folding and thrusting phase.
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Needed Ingredients
for Recognition of In
order to recognize
A 2-D projection of all of the tops of the Naricual Superior Santa Barbara field (Figure 2.1) shows anomalous alignments that have been outlined with different colors. The red alignment is centered around a depth of 14850 ft TVDss. Understanding the 2-D projection of Figure 2.2 is not straight forward. The anomaly is better understood on histogram as seen on Figure 2.4. However, that anomaly is just the tip of the iceberg. Figure 2.3 shows a 2-D projection of a group of wells that best express the geometry of the stratigraphic anomaly. Figures
2.5, 2.6, and 2.7 are examples of various types of evidence of detachments.
Detachment planes are linked to vertical compartmentalization and local
destruction of the reservoir quality below the major Drag folding is also commonly associated with detachments planes and is readily visible on inverted stacked Bischke plots (Figure 2.6). The two main detachment planes recognized in Santa Barbara seem to be also present in Carito and Furrial (Figure 2.7); that means that the lateral extent of these structural features is of the order of tens of kilometers.
Figure
3.1 illustrates a In Figure 3.3, thickness increase may be due to folding, but also thickness increase may be due to repetition linked to a detachment at 15900 ft TVDss. The cores show some 40 meters of intense fracturing around the detachment plane at 15900 ft TVDss. Note the high density of low angle and horizontal open fractures.
Massive Deformation Bands Associated with Single Detachment Planes
All
well locations and depths in Figure 4.1 correspond to official-faults,
previously interpreted as individual faults. Our 3-D analysis indicates that
these Systematic features of the reservoir interval are oil-bearing porous sands with very sharp boundaries, quartz cemented tight sands, and slickensided lower boundary. The cored well (Figure 4.3) shows diagenetically altered sandstone with regular spacing. Each of the selected photos shows the systematic existence of a slickensided surface below a tight sandstone with prominent quartz cement. Dissolution features are common, including stylolites and horsetail stylolites.
All
of the wells in the area of interest in Santa Barbara field (Figure
5.1) have
encountered faults at depths very similar when considering that the overall
reservoir thickness is of the order of 3000 feet (Figure
5.3). These The
A
birdseye map (Figure 5.5) was generated using seismic, dipmeter data, and the
stratigraphy of the layers recognized below the hypothetical Following a compressive phase from the northeast associated with a low angle thrust and its related folding, another compression has led to a partial decapitation of the existing fold (Figures 5.7 and 5.8). The low angle to horizontal detachment planes have been associated with some 400 feet of highly reduced reservoir quality below the 14850 ft detachment plane wherever sand is present above the plane. Where Carapita shales overlie the detachment plane, there is little damage to the underlying reservoir. Other Vital Pieces of Information
Seismic has been of limited help for the interpretation of the Santa Barbara Field. This is due to a combination of factors, including acquisition, processing, and structural complexity (Figure 6.1). All of the production data had to be used to give a reliable model. Visualization has been essential to get the maximum integration with the petrophysicists, geochemists, and especially the reservoir engineer. It has been necessary to make maximum use of 3-D visualization throughout the various phases of interpretation/integration and to make frequent comparisons with outcrop analogues. Geochemistry,
petrophysics, and reservoir engineering have been of great help in the
structural interpretation. The diagram in Figure 6.2 shows the variability of
oil gravity with respect to depth. The API data fully support the Recognition
of
Parnaud, F., Y. Gou, J. -C. Pacual, I. Truskowski, O. Gallango, H. Passalacqua, and F. Roure, 1995, Petroleum Geology of the Central Part of the Eastern Venezuelan Basin, in Petroleum Basins of South America: AAPG Memoir 62, p. 741 – 756. Thanks to PDVSA Oriente and PDVSA Intevep for the permission to present this paper. |