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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Main Controlling Factors of Sedimentary System of Paleogene Qi’nan Fault-Step Zone, Bohai Bay Area
(1) Exploration & Production Research Institute, CNOOC LTD.-TianJin Bohai Oilfield, Tianjin, China.
Qi’nan fault-step zone sited at south of Qikou depression, is one of the earliest petroleum exploration areas in Bohai Bay. But in recent years, sedimentary and reservoir characteristics of Paleogene puzzled the geologists and engineers. Based on the sequence stratigraphy, from the data of well drilling, logging and heavy mineral analysis, this paper focused on the analysis of Paleogene sandstone distribution, sedimentary systems and their controlling factors, then predict the favorable prospect of this area.
Drilling revealed strata between Es3 to Ed1formation in Qi’nan fault-step zone can be divided into 2 second-order sequences, a total of 8 third-order sequences. There developed braided fluvial-delta system, meandering fluvial-delta system, fan-delta and lacustrine fan system and so on. According to the distribution of sediments source there are four sand dispersion systems, namely Chengzikou, Shaleitian, Shazhong lower uplift and Yanshan.
Palaeogeomorphology is the key factor controlling the sand bodies’ spreading, which mainly presents in three aspects: 1. Paleo-structural framework controlling the facies differentiation. Different facies were controlled by fiffrent fault-steps respectively; 2. Structural transform belt controlling the mainstream of the paleo-water system, the upthrown block between the large faults conversion formed the lowland inheritance structure; 3.Fault-slope controlling the sand accumulation. Fault-ramp belts formed along the long active large faults of Hai-1, Hai-3 and QK18-1 are the major sand-rich areas.
Sand vertical distribution is controlled by the compound base-level cycle. The second-order base level cycles of the continental rift lake are controlled by the episodic rifting basin. The third-order base level cycles are mainly controlled by the pulse of basin marginal faults. As a whole, the second-order base level gives the trends of sand body spreading, the third-order base level reveals the relatively position where sands focused. The combination of the two types can better reflects disciplines of the sand bodies’ spreading.
The research indicates that the slope belt from Hai-1 fault nearby to western Qikou 17-1 and the third-level step zone of southern Hai-4 fault were the main potential areas, while the former is the right area seeking for deep natural gas resources.