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Mark A. Beeunas1, Thomas A. Hudson1, Josann A. Valley2, David K. Baskin2, Wesley Y. Clark1
(1) Chevron Production Co, New Orleans, LA
(2) Chevron Research and Technology Co, New Orleans, LA

Abstract: Identification of reservoir discontinuities using hydrocarbon compositional analyses, Genesis field (Green Canyon 205), Gulf of Mexico

Detailed chromatographic analysis of reservoir fluids collected during exploration, delineation, and production provide key data for reservoir modeling and surveillance. Reservoir discontinuities and compositionally graded oil columns at the Genesis Field were detected by this methodology.

Oil compositions are measured using gas chromatography and compared using software that identifies diagnostic chromatogram peak ratios. The application of hydrocarbon compositional analysis for continuity evaluation is based on the premise that oil samples from a single hydrocarbon accumulation will have similar compositions, whereas separate hydrocarbon columns and graded hydrocarbon columns will usually have discernibly different compositions.

Results of hydrocarbon compositional analyses from the Genesis Field supported the pre-production interpretation of discontinuities within the N1 reservoir. A suspected transmissibility restriction in the N1 sand was indicated by pressure transient data that correlated to an area of lower 3D-seismic amplitude with minor vertical offset. Significant compositional differences of the hydrocarbons from opposite sides of this feature agreed with the inferred presence of a reservoir barrier. This conclusion is supported by current compositional and production data.

Interpretation of compositional data is complicated by the possibility of compositionally graded oil columns. A graded column was identified in the N3L sand by progressive changes in the composition of produced oil over time. Although these data were attributed initially to reservoir separation, pressure data and the evolution of the oil composition towards that of downdip wells indicate good communication within the reservoir. These changes may reflect the gradual movement of the oil-water contact toward the wellbore during production.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana