New understanding on development conditions of lacustrine source rocks and characterization of high-quality source rocks in Songxi Sag, Qiongdongnan Basin

Abstract

With respect to the problems of unclear development mechanisms of source rocks and distribution of high-quality source rocks in the Songxi Sag, Qiongdongnan Basin, medium-deep lacustrine source rocks in eastern and western subsags of the Songxi Sag were predicted, according to geochemical characteristics of crude oil from drilled wells, sedimentary characteristics of these subsags, the characteristics of seismic facies and attributes, and comparison and analysis of seismic velocity spectrum. In this study, it was defined that the following three factors, namely, subsidence rate of the Songxi Sag, injection points in the provenance, and sag structure, coupled and controlled the development and distribution of lacustrine source rocks. Western subsag of the Songxi Sag was adjacent to large-scale injection points in the provenance as sand-rich sediments, and then led to low paleo-productivity level, high depositional rate, and open and oxidized water body setting. It had no fundamental conditions for the formation of high-quality source rocks. Eastern subsag was far from these large-scale injection points with the development of only small sand bodies, which was favorable for the development of medium-deep lacustrine source rocks. Geochemical indexes of crude oil from drilled wells indicated that the crude oil in the study area came from medium-deep lacustrine source rocks, which confirmed the occurrence of good source rocks in the Songxi Sag. Eocene medium-deep lacustrine source rocks had seismic characteristics of low frequency, continuous and strong reflectance in addition with low velocity anomalies that were mainly distributed in the center of eastern subsag and gentle slope zone. Above-mentioned understandings could guide well allocation in eastern subsag. Eocene oil shale was firstly encountered in the wells, Qiongdongnan Basin. These source rocks had good indexes and enhanced oil exploration prospect in the Songxi Sag, Qiongdongnan Basin.

Keywords

• high-quality source rocks
• infill pattern
• seismic characteristics
• songxi sag
• qiongdongnan basin