The origin, type and hydrocarbon generation potential of organic matter in a marine-continental transitional facies shale succession (Qaidam Basin, China)

Guo-Cang Wang,Li Tang,Min-Zhuo Sun,Shu-Fang Gao

doi:10.1038/s41598-018-25051-1

Abstract

This organic-rich shale was analyzed to determine the type, origin, maturity and depositional environment of the organic matter and to evaluate the hydrocarbon generation potential of the shale. This study is based on geochemical (total carbon content, Rock-Eval pyrolysis and the molecular composition of hydrocarbons) and whole-rock petrographic (maceral composition) analyses. The petrographic analyses show that the shale penetrated by the Chaiye 2 well contains large amounts of vitrinite and sapropelinite and that the organic matter within these rocks is type III and highly mature. The geochemical analyses show that these rocks are characterized by high total organic carbon contents and that the organic matter is derived from a mix of terrestrial and marine sources and highly mature. These geochemical characteristics are consistent with the results of the petrographic analyses. The large amounts of organic matter in the Carboniferous shale succession penetrated by the Chaiye 2 well may be due to good preservation under hypersaline lacustrine and anoxic marine conditions. Consequently, the studied shale possesses very good hydrocarbon generation potential because of the presence of large amounts of highly mature type III organic matter.

Highlights

Because of the successful commercial exploration and development of shale gas in the USA1–3, large investments in the exploration and production of shale gas, an alternative hydrocarbon resource, have been made in China in recent years[4,5]
Type and hydrocarbon generation potential of shale, we studied the characteristic of its insoluble organic matter by Rock-Eval to determine the Total Organic Carbon content(TOC), free hydrocarbons (S1), hydrocarbon generative potential (S2), temperature (Tmax) at themaximum of the S2 peak, and production index (PI = S1/(S1 + S2)), hydrogen index (HI = S2/total organic carbon (TOC) × 100)
The organic matter within these rocks originated in a marine shelf depositional environment

Summary

Introduction

Because of the successful commercial exploration and development of shale gas in the USA1–3, large investments in the exploration and production of shale gas, an alternative hydrocarbon resource, have been made in China in recent years[4,5]. Despite the relatively numerous oil exploration studies in this area[12,13,14], few studies on the organic geochemical characteristics or hydrocarbon generation potential of the marine-terrigenous facies shale have been published[15]. Cao (2016) analyzed the type, amount and maturity of the organic matter on the marine-terrigenous facies shale and mainly studied characteristics and factor of shale gas in this area[15], but origin of the organic matter and hydrocarbon generation potential were not reported. The Chaiye 2 well is located within the Wulan tectonic belt and penetrates the upper Carboniferous Keluke formation This unit is mainly composed of dark shale, mudstone, carbonaceous shale and coal[16,17] (Fig. 1)

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