Lithofacies and depositional environment of D sequence, Upper Oligocene formation of Gao field, Cuu Long basin from well log data

Abstract

The D sequence, Upper Oligocene formation is the main target for petroleum exploration and exploitation activities at GAO field in Cuu Long basin. The study primarily uses well log and core samples data from exploration wells in the area to investigate the lithofacies and depositional environment. Shale, sandstone and volcanic material were identified in the gamma ray log. Base on variations in lithology as indicated by well log response, the D sequence can be divided into three members (D1, D2, and D3 sub-sequences), which were deposited in fluvial to lacustrine environments. The shapes of the gamma ray signature of D2 sub-sequence are serrated funnel, bell and cylindrical reflected sediment deposited in a mouth bar and a distributary channel environment. Sedimentary structure characterized by parallel ripple cross-bedding, with occasional weak cut cross-bedding or small lenticular features. Meanwhile, sediments of D1 and D3 sub-sequences were deposited in a distal lacustrine environment, characterized by saw tooth features, indicating thick layers of lacustrine shale interbedded with thin shore face sand layers