Key reservoir parameter for effective exploration and development of high-over matured marine shales: A case study from the cambrian Niutitang formation and the silurian Longmaxi formation, south China

Abstract

Success has been achieved in the exploration and development of marine shales of the Lower Silurian Longmaxi Formation in southern China, but there has been almost no commercial shale gas produced from the Lower Cambrian Niutitang Formation in much of the same area. Breakthroughs have been made with shale gas in the Niutitang Formation in Yichang City, south China, by the China Geological Survey (CGS). How to evaluate and exploit the Niutitang shale more quickly, accurately and efficiently remains a major question. This paper proposed that the key reservoir parameter for effective exploration and development of marine shales is based on the thermal maturity. Shale samples from Chongqing City of both the Longmaxi Formation and the Niutitang Formation were selected for the analysis of total organic carbon (TOC) content, mineral composition and equivalent vitrinite reflectance (Eq-Ro), as well as for the characterization of organic matter pores (OM pores) using focused ion beam scanning electron microscopy (FIB-SEM). Selected shale samples from Yichang City of the Niutitang Formation were also studied for comparison. Results show that shale samples from Chongqing present a large number of OM pores in the Longmaxi Formation, but almost no OM pores in the Niutitang Formation. While shale samples from Yichang present a large number of OM pores in the Niutitang Formation. Commercial gas has been produced from shale reservoirs with abundant OM pores, such as the Longmaxi shale in Chongqing and the Niutitang shale in Yichang. By contrast, there is no gas production from shale reservoirs with no OM pores as is the case for the Niutitang shale in Chongqing. Thus OM pores is an indicator for the evaluation of marine shale gas potential. Because these shales have similar TOC content, mineral composition and kerogen type, but different thermal maturity, OM porosity is mainly controlled by the thermal maturity and the key geological parameter for shale reservoir potential is thus a suitable thermal maturity. Marine shale reservoirs favorable for gas production generally have an Eq-Ro ranging from 2.0% to 3.0%, which is mainly controlled by ancient burial depth of reservoirs. This means exploration and development of highly over matured marine shales should be focused on less buried areas such as the edge of paleo-uplift.