Diagenesis and hydrocarbon emplacement in the Upper Triassic Yanchang Formation tight sandstones in the southern Ordos Basin, China

ABSTRACT

The Upper Triassic Yanchang Formation in the southern Ordos Basin is a tight sandstone but productive hydrocarbon exploration target. A variety of analyses, including thin-sections, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), fluorescence, isotope and fluid inclusion, have been used to analyse the reservoir petrology and diagenesis to understand the evolution of reservoir porosity. The sandstones are mostly feldspathic litharenites and lithic arkoses with low porosities (7.6% on average) and extremely low permeabilities (0.49 mD on average). A complex diagenetic alteration history of the reservoir caused different kinds of reduction in reservoir porosity. The carbonate cements are sourced from adjacent organic-rich mudstone and precipitated with a higher content near the edges of sandstone units and a lower content at the centres of sandstone units, resulting in two different diagenetic evolution paths. The analysis of porosity evolution history and hydrocarbon emplacement produced the following conclusions: (1) during the eodiagenesis period, mechanical compaction reduced the primary porosity dramatically from 40 to 19%; and (2) during the mesodiagenesis period, the first-phase oil charged the reservoir (porosity ranging from 14 to 19%) as bitumen along detrital grains forming a preferential pathway for subsequent oil emplacements. In the second-phase of oil charge, the reservoir porosity ranged from 9 to 14% limiting water–rock reactions. During the third-phase of oil charge, porosity of the reservoir remained at 9%. In general, the densification period (with a porosity close to 10%) of the reservoir occurs at the same time as the major oil emplacement and may be an explanation for the low oil saturation of the Chang 8 tight sandstone.

KEYWORDS:

• Ordos Basin
• tight sandstone
• diagenesis
• carbonate cements
• hydrocarbon charging
• porosity evolution

Acknowledgements

The research was funded by SINOPEC Exploration & Production Research Institute. The authors gratefully thank Professor Guiqiang Qiu, Associate Professor Renchao Yang and Dr Chunyan Liu. The authors thank Dr Liyuan Ma at SINOPEC Exploration & Production Research Institute for providing constructive suggestions. We also thank the North China branch of oil and gas, Sinopec, for providing data. Special thanks go to associate editor Dr Keyu Liu and the anonymous reviewers for their insightful reviews and constructive suggestions to make the paper better.

Disclosure statement

No potential conflict of interest was reported by the authors.