4D RESERVOIR GEOCHEMISTRY AS AN AID TO INTERPRETING PRODUCTION DYNAMICS, LEGENDRE FIELD, EASTERN DAMPIER SUB-BASIN

Abstract

Reservoir geochemistry is a low cost, field development/appraisal tool resting on the principle that fluids isolated by flow barriers show slight compositional and/or isotopic differences. Such differences reflect subtle variations in charge history related to the location of the source kitchen and the source rock maturity at the time of expulsion, as well as post fill processes such as water washing and leakage. High resolution gas chromatography (HRGC), multi-dimensional gas chromatography (MDGC) and compound specific liquid and gas isotope analysis (CSIA) were performed on a time series of fluids, comprising stored oil from two drill stem tests, and produced fluids from six points in the Legendre field, Dampier Sub-basin, to investigate changes in fluid composition as production proceeded. The Legendre field contains high gravity (46° API), low viscosity oil, hosted in two culminations (North and South) in a thin, high quality clastic reservoir of Berriasian age. Fluids from different wells within the Northern accumulation are indistinguishable, indicating the oil is in communication and no compositional gradient exists. By contrast, compositional and isotopic differences between fluids from the Northern and Southern accumulations demonstrate that these pools are not in communication, and should therefore be treated separately from a development planning perspective.

The differences in initial fluid compositions have been successfully used in conjunction with operational parameters to explain the increase in gas/oil ratio (GOR) of oil from Legendre South–2H that occurred after only 13 months of production. Comparison of pristine, preproduction separator samples with fluids collected after the observed increase in GOR, revealed that solution gas injected at Legendre West–1 has migrated rapidly into the southern part of the field. Integration of geochemical data with regional petroleum system concepts and a full 3D charge model has greatly assisted our understanding of these observations.