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AAPG Bulletin

Abstract

AAPG Bulletin, V. 100, No. 2 (February 2016), P. 191-212.

Copyright ©2016. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/11181514223

Effects of early petroleum charge and overpressure on reservoir porosity preservation in the giant Kela-2 gas field, Kuqa depression, Tarim Basin, northwest China

Xiaowen Guo,1 Keyu Liu,2 Chengzao Jia,3 Yan Song,4 Mengjun Zhao,5 and Xuesong Lu6

1Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China; present address: Department of Petroleum Geology, Faculty of Earth Resources, China University of Geosciences, No. 388, Lumo Road, Wuhan 430074, China; [email protected]
2Research Institute of Petroleum Exploration and Development (RIPED), PetroChina, Beijing 100083, China; Curtin University of Technology, GPO Box U1987, Perth, WA 6102, Australia; [email protected]
3RIPED, PetroChina, Beijing 100083, China; [email protected]
4RIPED, PetroChina, Beijing 100083, China; China University of Petroleum, Changping, Beijing 102249, China; [email protected]
5RIPED, PetroChina, Beijing 100083, China; [email protected]
6RIPED, PetroChina, Beijing 100083, China; [email protected]

ABSTRACT

Kela-2 is a giant gas field with a proven reserve of 597 tcf in the Kuqa depression, northern Tarim Basin. Widespread overpressures have been encountered in the Eocene and Cretaceous Previous HitsandstoneNext Hit reservoirs of the field, with pressure coefficients up to 2.1 Previous HitfromNext Hit drill-stem tests and well-log data analysis. Disequilibrium compaction associated with horizontal tectonic compression may be the dominant overpressure mechanism in the Previous HitsandstoneNext Hit reservoirs, because the overpressured Previous HitsandstoneNext Hit with a maximum burial depth over 6000 m (19,685 ft) displays anomalously high porosity and low density. The causes for Previous HitsandstoneNext Hit reservoirs with anomalously high porosity in the Kela-2 gas field were studied based on an integrated investigation of Previous HitsandstoneNext Hit reservoir characteristics, paleo oil–water contact, petroleum charge history, and overpressure evolution. Collective evidence indicates that early oil charge had retarded the porosity reduction of the reservoir Previous HitsandstoneNext Hit and resulted in disequilibrium compaction Previous HitfromNext Hit overburden rocks, and overpressure Previous HitfromNext Hit disequilibrium compaction and horizontal tectonic compression at the beginning of the rapid subsidence and deposition in the Kela-2 gas field again contributed to the preservation of the reservoir porosity: (1) overpressured mudstones in the Kela-2 gas field are characteristic of normal compaction, and overpressure was generated by horizontal tectonic compression instead of disequilibrium compaction; (2) the reservoir sandstones with high porosity and Previous HitpermeabilityNext Hit are associated with high paleo oil saturation, as indicated by quantitative grain fluorescence (QGF) responses and anomalous QGF on extract intensity; (3) Previous HitsandstoneNext Hit units below the paleo oil–water contact have very low porosity and Previous HitpermeabilityNext Hit; and (4) three episodes of oil and one episode of gas charge are identified in the Previous HitsandstoneNext Hit reservoirs of the Kela-2 gas field, and the later two episodes of oil charge occurred circa 5.5–4.5 Ma, which corresponds to the beginning of the rapid tectonic subsidence and deposition in the Kuqa depression. The initially charged oil in the Previous HitsandstoneNext Hit reservoirs was subsequently displaced by gas at circa 3–2 Ma through fault activation at the edge of the anticline trap. The overpressure evolution for the K1bs reservoir Previous HitsandstoneNext Hit in the Kela-2 gas field indicates that the apparent overpressure development in the Previous HitsandstoneNext Hit reservoir began at 5 Ma following the major oil charge and has been maintained to the present. Overpressure development Previous HitfromNext Hit 5 Ma in the Previous HitsandstoneNext Hit reservoirs of the Kela-2 gas field is believed to be the dominant cause of the porosity preservation.

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