Effectiveness of marine carbonate source rocks: a case study of Middle Triassic Leikoupo Formation in Western Sichuan Depression

Abstract

The Middle Triassic Leikoupo Formation (T2l) has been regarded as one of the most important strata for natural gas exploration in the Western Sichuan Depression of the Sichuan Basin. The main gas source has not been determined due to the controversy on the effectiveness of the T2l marine carbonate source rocks and exploration direction is breezing. Integrated studies on organic petrology and organic geochemistry indicate that, the organic macerals of kerogen in the T2l carbonate rocks in the Western Sichuan Depression are mainly consisted of sapropelic amorphous bodies. The total organic carbon (TOC) contents of both algal laminae and non-laminae sections insignificantly varied. The TOC content of the 3rd member of T2l is positively correlated with the content of clay minerals and negatively correlated with the content of carbonate minerals. If carboxylate is included to be calculated for these carbonates, the TOC content show increments no more than 0.2%. The T2l carbonate rocks in the Western Sichuan Depression mainly display low organic abundance and hydrocarbon potential, and only6.13% of the studied samples reach the standard values of effective source rocks, moreover, relative organic-rich layers are limitedly distributed. It is concluded that T2l marine carbonate rocks in the Western Sichuan Depression is unlikely to be main source rocks solely, and they can only contribute to the gas accumulation with a limited amount. Therefore, the migrating conditions is an important controlling factor to be considered for the T2l gas exploration, and the exploration of large-scale gas accumulation should focus on the areas with developed faults connecting the underlying Upper Permian source rocks.

Keywords

• carbonate rock
• source rock
• effectiveness
• total organic carbon content
• leikoupo formation
• western sichuan depression