Abstract: The Problem of Paleo-Oil-Water Contacts in Seismically Identifying Hydrocarbon Accumulations

Ware, Paul - Unocal and Carl Burgess - AIOC

Modeling has revealed a direct relationship between Amplitude Variation with Offset (AVO) class, porosity and fluid fill within a major Tertiary petroleum province. A swath of 3-D seismic data from a producing field was migrated to a target line through a well and full-waveform synthetic modeling was performed. Class 3 - 4 AVO response (high normal incidence reflectivity with low positive or negative gradient) is expected at the main productive reservoirs. However, reprocessing of the 3-D seismic survey also revealed a secondary amplitude anomaly considerably downdip of the known oil-water contact (OWC). A well drilled between these two anomalies showed residual hydrocarbons (HCs) in this zone. A thick, HC-bearing shale underlies this structural trend. Pliocene compression, plus sedimentary loading of this on one flank of the structure, initiated argillokinesis and uplift, which created a linear sill. This, in turn, caused further asymmetric loading, and oversteepening of the northern flank. Finally, the diapir rose to a level at which explosive ex-solution occurred within the shale and dissolved gases penetrated through the overlying formations. HCs then leaked to the surface, either directly through the mud volcano diatremes or via faults that became active at this time, until the migration conduit was sealed by fine clastics. An amplitude anomaly remains at the position of the "paleo-OWC" (see Fig. 1 & 2). Satellite data and regional 2-D seismic show clearly that most of the structures in the basin that exhibit mud volcanism are currently leaking HCs to the sea surface. Regional 2-D seismic lines show that the am plitude anomalies in the main productive zone correlate with those in an adjacent parallel trend of structural closures. AVO modeling of this zone shows Class 3 - 4 responses. Other amplitude anomalies on these sections also fit well with the model. The coarser clastics that were ponded in the syncline to the north of the sill exhibit higher amplitudes due to their higher porosity. Near-surface amplitude anomalies are the result of shallow gas that has migrated vertically up faults and then laterally up-dip. Several exploratory wells have been drilled in these structures. Preliminary results have been mixed, including costly dry holes. Geochemical modeling of the surrounding synclines has shown that a functional petroleum system exists and that HCs should have migrated into the structures. There is, therefore, a strong likelihood of subsequent leakage. As the intervening syncline developed (due to loading and shale withdrawal) the northern flanks of the southern structure became oversteepened, with the effect of flattening the reverse faults. To the north, however, significant reverse faulting never extended into the productive section. As the syncline grew, "keystone" (normal) faults formed over the crest of one structure to the northeast. These provided a conduit for entrapped HCs to escape. By contrast, a seismic line that extends over a structure to the northwest shows no "keystone" faults. AVO modeling shows two Class 3 - 4 anomalies. Amplitude mapping of a proprietary 3-D seismic volume shows these to be within structural closure. An exploratory well tested 3500bopd from the upper zone and 22.5 MMcfgpd from the lower as well as significant condensate. A shallow amplitude anomaly is illustrated that appears to coincide with a regionally pervasive anhydrite layer. Amplitude mapping, however, shows that the higher amplitudes are structurally constrained and have the 3D geometry of channels and over-bank deposits. Other plays exist within the area, both structural and stratigraphic. Climatic changes have created periodic pulses of sedimentation and exploration fairways that can be identified through sequence stratigraphy. On-going tectonic compression has created uplift and the development of a foreland basin, which has overlain a Plio-Pleistocene regional tilt to the region. Experience has already demonstrated that amplitude anomaly mapping that does not take account of the regional geology will surely lead to more expensive dry holes in this area.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil