--> New Method to Evaluate Carbonate Cap Rocks: Enlightenment from Rock Mechanical Property

2018 AAPG International Conference and Exhibition

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New Method to Evaluate Carbonate Cap Rocks: Enlightenment from Rock Mechanical Property

Abstract

Marine petroleum resource has recently been one of the focal points for oil and gas exploration in China. Marine strata in China was formed at an early age and experienced several tectonic changes, in addition to source rocks and reservoir, the effectiveness of the cap rocks is one of the key factors in hydrocarbon accumulation, and will affect the evaluation of marine petroleum resource as well as the determination of exploration direction in China. Most researches have concentrated on the traditional quality cap rocks, such as evaporite rocks and mud shale; whereas little study has been done on the effectiveness of carbonate cap rocks. In order to unravel sealing mechanism of carbonate cap rock, 5 carbonate rock samples in North Slope of Tazhong uplift in Tarim basin were selected, they are from Yingshan Formation which is in upper part of Lower Ordovician and at the same depth in the same well. The critical confining pressure of brittle to ductile transition was obtained by using triaxial compression rock mechanical test. Sonic velocity of rock samples before and after triaxial compression experiment were tested respectively. X ray diffraction and rock CT scan methods were employed to understand the characteristic and micro structures of fractures in brittle and ductile state respectively and analyze the sealing capacity affected by such difference. The results are listed as follows: 1) the critical confining pressure of carbonate rocks of Yingshan Formation from brittle to ductile is calculated as 74.16Mpa which goes to the depth of 6249m; 2) the content of calcite and quartz in carbonate rocks affects the depth of brittle to ductile transition; 3) wave impedance value of rock in ductile deformation is bigger than that in brittle deformation; 4) rock CT scan shows that micro pores of sample contract when they are in ductile deformation. In addition to physical property of the carbonate rock itself, micro fractures generated by structural events is an important factor that affect the effectiveness of carbonate cap rocks. We analyzed the experiment results integrated with comprehensive static micro sealing capacity examination of carbonate cap rocks and conclude that the sealing mechanism of carbonate rock is in ductile deformation state, micro factures are not or less developed in rock samples, micro pores of rock sample contract result in the process of densification of carbonate rocks.