Method for identifying effective carbonate source rocks: a case study from Middle\u2013Upper Ordovician in Tarim Basin, China

Jun-Qing Chen,Zhi-Peng Huo,Luo-Fu Liu,Kui-You Ma,Mei-Ling Hu,Bo Pang,Song Wu,Xiong-Qi Pang,Zhuo-Heng Chen

doi:10.1007/s12182-020-00489-z

Abstract

Hydrocarbon expulsion occurs only when pore fluid pressure due to hydrocarbon generation in source rock exceeds the force against migration in the adjacent carrier beds. Taking the Middle–Upper Ordovician carbonate source rock of Tarim Basin in China as an example, this paper proposes a method that identifies effective carbonate source rock based on the principles of mass balance. Data from the Well YW2 indicate that the Middle Ordovician Yijianfang Formation contains effective carbonate source rocks with low present-day TOC. Geological and geochemical analysis suggests that the hydrocarbons in the carbonate interval are likely self-generated and retained. Regular steranes from GC–MS analysis of oil extracts in this interval display similar features to those of the crude oil samples in Tabei area, indicating that the crude oil probably was migrated from the effective source rocks. By applying to other wells in the basin, the identified effective carbonate source rocks and non-source rock carbonates can be effectively identified and consistent with the actual exploration results, validating the method. Considering the contribution from the identified effective source rocks with low present-day TOC (TOCpd) is considered, the long-standing puzzle between the proved 3P oil reserves and estimated resources in the basin can be reasonably explained.

Highlights

In the past, scholars in the world have put forward different definitions of effective carbonate source rock
We propose a method for identifying effective source rock in carbonates using mass balance approaches by quantifying hydrocarbon expulsion and use the Middle–Upper Ordovician source rock of Tarim Basin in China as a case study to illustrate the procedure and demonstrate the feasibility of the proposed method
Increase in depth leads to a rise in formation temperature, which leads to a decreasing trend of crude oil density (Fig. 3a)

Summary

Introduction

Scholars in the world have put forward different definitions of effective carbonate source rock. Pang et al (1993) considered only those rocks that expel freephase hydrocarbons in large quantities are effective source rocks. Only those rocks that contain sufficient organic matter (quantity) with good kerogen type (quality) at a certain thermal evolution stage (maturity) and that are capable of expelling sufficient hydrocarbons for forming commercial accumulations, are referred as effective source rocks. Depending on quality and maturity, the threshold of TOC value as an effective source rock varies. For the convenience of discussion, we take 0.5% as the threshold value of present-day TOC (TOCpd) to define the high organic matter and low organic matter carbonate source rocks.

Methods

Results

Discussion

Conclusion