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ABSTRACT: Controls on Reservoir Quality in the Giant Yacheng Gas Field, South China Sea

S. Bloch, C. D. Atkinson

Middle Tertiary sandstones from the giant Yacheng field, South China Sea, were derived from a basement uplift, and range in composition from lithic arkoses to feldspathic litharenites. The effect of chemical diagenesis on the framework composition is relatively limited, as suggested by the compositional similarity between uncemented samples and samples completely cemented by early calcite. Calcite cementation, however, is not widespread. Reservoir quality and its distribution in the sandstones is primarily a function of textural and mineralogical parameters related to deposition in different types of paleoenvironments. The paleoenvironments range from basement regoliths and proximal fan-deltaic sediments deposited in lacustrine settings through increasingly more marine, t dally influenced distal fan-delta braided streams and estuarine channels, and eventually littoral beaches and offshore sublittoral bars.

A petrologic classification was developed which relates rock composition to depositional environments which explains observed variations in reservoir quality. For a given burial history, there is an excellent correlation between permeability and porosity (dependent variables) and the following depositional facies-controlled, independent physical variables: grain size, sorting, and abundance of matrix and ductile grains. This correlation, in conjunction with a predictive facies distribution model, can be used to predict reservoir quality in future wells in the Yacheng field.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990