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AAPG Bulletin, Preliminary version published online Ahead of Print 15 September 2022.

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Impact of diagenesis on the pore evolution and sealing capacity of carbonate cap rocks in the Tarim Basin, China

Jun Wu123 , Tailiang Fan12 , Enrique Gomez-Rivas3 , Anna Travé3 , Qian Cao4 , Zhiqian Gao12 , Shanshan Wang5 , and Zhihong Kang12

1 School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
2 Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, Beijing 100083, China
3 Department of Mineralogy, Petrology and Applied Geology, Faculty of Earth Sciences, University of Barcelona (UB), Barcelona 08028, Spain
4 Exploration and Development Research Institute of PetroChina Changqing Oilfield Company, Xi’an 710018, China
5 Beijing Municipal Commission of Planning and Natural Resources Yanqing Branch, Beijing 102100, China

Ahead of Print Abstract

Analyzing the pore structure and sealing efficiency of carbonate cap rocks is essential to assess their ability to retain hydrocarbons in reservoirs and minimize leaking risks. In this contribution, the impact of diagenesis on the cap rock’s sealing capacity is studied in terms of their pore structure by analyzing rock samples from Ordovician carbonate reservoirs (Tarim Basin). Four lithology types are recognized: highly compacted peloidal packstone-grainstone, highly cemented intraclastic-oolitic-bioclastic grainstone, peloidal dolomitic limestone, and incipiently dolomitized peloidal packstone-grainstone. The pore types of cap rocks include microfractures, intercrystalline pores, intergranular pores, and dissolution vugs. The pore structure of these cap rocks was heterogeneously modified by six diagenetic processes, including calcite cementation, dissolution, mechanical and chemical compaction, dolomitization, and calcitization (dedolomitization). Three situations affect the rock’s sealing capacity: (1) grainstone cap rocks present high sealing capacity in cases where compaction preceded cementation; (2) residual microfractures connecting adjacent pores result in low sealing capacity; (3) increasing grain size in grainstones results in a larger proportion of intergranular pores being cemented. Four classes of cap rocks have been defined according to the lithology, pore structures, diagenetic alterations, and sealing performance. Class I cap rocks present the best sealing capacity because they underwent intense mechanical compaction, abundant chemical compaction and calcite cementation, which contributed to the heterogeneous pore structures with poor pore connectivity. A four-stage conceptual model of pore evolution of cap rocks is presented to reveal how the diagenetic evolution of cap rocks determines the heterogeneity of their sealing capacity in carbonate reservoirs.

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Please cite this AAPG Bulletin Ahead of Print article as:

Jun Wu , Tailiang Fan , Enrique Gomez-Rivas , Anna Travé , Qian Cao , Zhiqian Gao , Shanshan Wang , Zhihong Kang: Impact of diagenesis on the pore evolution and sealing capacity of carbonate cap rocks in the Tarim Basin, China, (in press; preliminary version published online Ahead of Print 15 September 2022: AAPG Bulletin, DOI:10.1306/11082120136.