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Abstract

AAPG Bulletin, V. 99, No. 4 (April 2015), P. 763ndash789.

Copyright copy2015. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/11051414018

Heterogeneous distribution of pyrobitumen attributable to oil cracking and its effect on carbonate reservoirs: Feixianguan Formation in the Jiannan gas field, China

Pingping Li,1 Fang Hao,2 Baiqiao Zhang,3 Huayao Zou,4 Xinya Yu,5 and Guangwei Wang6

1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China; [email protected]
2Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China; [email protected]
3Jianghan Oil Field Branch Company, Sinopec, Qianjiang, Hubei 433124, China; [email protected]
4State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China; [email protected]
5State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China; [email protected]
6State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China; [email protected]

ABSTRACT

Few previous studies have focused on solid bitumen distribution and its effect on gas reservoir quality during oil cracking. Solid bitumen is commonly found in both gas and nongas reservoirs of the Triassic Feixianguan Formation BLTN14018eq1 in the Jiannan gas field. The BLTN14018eq2 natural gases are mainly secondary cracking gases of oil generated from source rock of the Permian Wujiaping Formation BLTN14018eq3, and the reservoir experienced temperatures above 150°C (302°F) for about 35 m.y. A relatively narrow range of BLTN14018eq4 values and a wide range of BLTN14018eq5 values and widespread solid bitumen indicate that oil cracking took place in the gas field. Low concentrations of BLTN14018eq6 (commonly <0.81%) suggest that high-reflectance (2.57%–3.07%) solid bitumens are pyrobitumens, which would have been mainly derived from oil cracking. Gases preferentially occupy larger pore spaces, and oil is displaced into small pores and throats by overpressure during oil cracking. In this way, pyrobitumens can reduce the magnitude of porosity in relatively tight reservoirs. Moderate-quality oil reservoirs (paleoporosity 2.2%–8.0%) are between or adjacent to high-quality oil reservoirs and are probably poor-quality or nongas reservoirs after oil cracking. Carbonate reservoirs (paleoporosity >8.0%) can be high-quality gas reservoirs after oil cracking and should be favorable targets for future gas exploration in the northeastern Sichuan Basin and adjacent areas.

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