Due to the great potential for future petroleum resource discoveries, deeply buried (> 3,500 m) carbonate reservoirs are currently hot exploration targets in many of China's sedimentary basins.

The roles of diagenesis impacts in carbonate reservoir development are still poorly understood, hampering the world-wide petroleum exploration and production planning processes.  

Associate Professor JIANG Lei and his colleagues from the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences found that deeply buried dolostone appeared to have much better petroleum reservoir potential when compared with limestone. Their findings were published in Geochimica et Cosmochimica Acta and Marine and Petroleum Geology.  

In the research, each diagenetic event's impact on the quality of the reservoirs was further quantitatively evaluated, either from data counting and/or the diagenetic modelling results. Oolitic limestone and dolostone core samples from the Early Trassic Age were collected from the shoal facies of the eastern Sichuan Basin.  

A combination of petrology, fluid-inclusion microthermometry, stable and radiogenic-isotopes, and point counting, along with the modelling of the diagenesis, pressure, porosity, and permeability, was used to constrain the diagenetic impacts on reservoir development. When compared with limestone, with an average porosity of < 2% due to early calcite emplacement, dolomstone has much better reservoir quality (with an average porosity value of approximately 10%).  

Therefore, dolomitization preserved more porosity from calcite precipitation by compaction. The diagenetic modelling results showed that the TSR had enhanced the porosity by approximately 1.6%, which led to a reservoir permeability increase from 110 mD to 264 mD, as supported by the permeability modelling.

The fluid pressure modelling results showed that overpressure had occurred and was perhaps related to the TSR, which may lead to the maintenance of the porosity due to the inhibition of the compaction processes.  

The results of this study were the first to quantitatively answer why deeply buried dolostone had better reservoir quality when compared to the limestone countparts, and also clarified the role of the TSR impacts on the quality of carbonate reservoirs.  

This study was conducted in collaboration with the University of Liverpool, the University of Texas at Austin, and the PetroChina Hangzhou Research Institute of Geology.  

The research was supported by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the National Science and Technology Major Project, the Office of the China Postdoctoral Council, and the Chinese Scholarship Council. 

Figure 1. Core and photomicrographic images show that there is abundant vuggy porosity (in blue) present in the oolitic dolostone, whereas the porosity in the oolitic limestone was very low due to heavy calcite cementation. Lower Triassic Feixianguan Formation, Sichuan Basin, China. (Image by JIANG)