STRATIGRAPHIC FRAMEWORK AND PETROLEUM GEOCHEMISTRY OLIGOCENE SOURCE ROCKS, NORTHERN SONG HONG BASIN

Abstract

The lacustrine oil-prone organic-rich mudstones in the Oligocene are the most important source rocks which have contributed to generate significant hydrocarbon accumulations in the northern Song Hong basin but still poor understanding due to the limitation of data. Newly c. 800km2 3D seismic data and well data allows investigating the stratigraphy and geochemistry of the northern Song Hong basin in detail to define the stratigraphic framework and hydrocarbon potential. The area is characterized by three relatively elongate grabens which are separated by the northeast-southwest basement faults. The most completed synrift sequence is observed in the central graben and is divided into three distinct sections, lower section (Ol), middle (Om) and upper (Ou), corresponding to three phases defined by tectonic systems tracts, from rift initiation to rift development and finally fill up phase. Uplift at the end of Oligocene has strongly modified the syn-rift sequences, resulting in folding, reactivated some basement faults and generated a series of new EW trending faults which may act as potential structural traps in the area. Organic geochemical analysis of the lacustrine mudstone samples from Ol and Om section of well D-1X shows fair to high organic carbon contents, Total Organic Carbon (TOC) 0.51 - 4.86 wt% and Hydrogen Index (HI) 273 - 552 mg HC/g TOC, corresponding to the type II kerogen. Biomarkers indicate the contribution of algal, high land plant, and bacterial sources to the composition of organic matters. Organic matters were deposited in the less oxic environment, in the fresh-water to brackish marine influence.