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Controls on Fluid Quality Variations Across Four Producing Fields in the Deepwater Gulf of Mexico

Abstract

BHP Billiton has working interest in four deepwater fields (Mad Dog, Atlantis, Shenzi, and Neptune) in the Atwater Foldbelt region of the Gulf of Mexico. These fields have been on production for close to 10 years and are considered to be mid- to late-life assets. A fully integrated geological study was launched to maximize production and provide insight for infill opportunities. The products of this study have enabled us to address technical issues common to all four fields. A key component of the study was to gain an understanding of fluid quality variations across the fields and on an intra-field scale. Fluid quality variations are a key input to field development and production planning. The four fields are part of a common petroleum system sourced from the Late Jurassic (Tithonian) source interval. All feature Early - Mid Miocene clastic reservoirs - as well as similar trap styles, seal mechanisms, and inferred migration pathways. Despite this commonality, fluid quality (GOR, API Gravity, viscosity) varies significantly between and within the fields. Our approach utilized paired fluid PVT and geochemistry analyses to develop and test our understanding of the controls on fluid quality. Reservoir fluid geochemistry and PVT data revealed sub-regional changes in source character (clastic input) and maturity from SW to NE. The predictable sub-regional changes are overprinted by biogenic gas mixing and biodegradation to produce the observed patterns of fluid quality. The relative importance of each factor varies between fields, further complicating our interpretations of the controls on fluid variations. The products of the Atwater Foldbelt petroleum systems analysis have already been used to make business decisions such as predicting fluid quality in undrilled compartments, for use in production planning, explaining phase behavior in a producing field and for identifying in-fill opportunities. Our work shows the value of integrated petroleum systems analysis in a development and production environment.