Following the sequence stratigraphy theory established by Vail PR et al., date of 54 exploration wells, 2D and 3D seismic interpretation, and regional tectonic evolution, sequence stratigraphy framework of the Ordovician... [ view full abstract ]
Following the sequence stratigraphy theory established by Vail PR et al., date of 54 exploration wells, 2D and 3D seismic interpretation, and regional tectonic evolution, sequence stratigraphy framework of the Ordovician carbonate rocks in the Tazhong area, Tarim Basin, was carried out. The investigated stratigraphic series could be identified 11 sequence boundaries (SB1-SB11). SB1-SB4 and SB6-SB11 were eroded unconformity surface, SB5 was through the weathering denudation, leading the sequence missing of Yijianfang formation, therefore the SB5 belong to the tectonically pressed unconformity surface. There can be divided 10 three-order sequences (SQ1-SQ10). Each sequence comprise a regional depositional cycles from transgression to regression. In late of early Ordovician (SQ1), Tazhong area had relatively stable tectonic setting, base-level changed from rising to falling slowly, and mainly developed carbonate platform with dolomite sedimentary. From early Ordovician to middle Ordovician (SQ2–SQ4), Tazhong area experienced continuous structure uplift, and base-level fell rapidly. In this time, the accommodation space diminished gradually, and the study area mainly developed beach with oolites and intraclastic graistones. With the base-level falling, the Lower-Middle Ordovician had undergone erosion during the middle Ordovician, which resulted in the most important unconformity on the Yingshan Formation’ top. After the Tazhong uplift forming, the tectonic setting tended to be relatively stable again in the late Ordovician. During early of late Ordovician (SQ5–SQ6), the base-level changed from falling to rising slowly, and Tazhong area started to underwent transgression. At the turning position from falling to rising of the base level, huge thick shaly limestone section deposited, which is the main hydrocarbon source rock and favourable caprock (Liang 3-5 section). After this period, the base-level rose fast and the transgression reached its climax during the middle of late Ordovician (SQ7 – SQ9). With high-energy hydrodynamic depositional conditions in this time, the Tazhong area mainly developed organic reef and particles beach. Due to the above-mentioned large scale sea-level rise with the large area of transgression, the carbonate platform of Tazhong area was flooded, and deep shelf-basin developed in the late of late Ordovician (SQ10). Associated with the base-level cycles and accommodation space variations, ten reservoir-caprock assemblages could be distinguished around the sequence boundaries, the reefs or beach deposits as a reservoirs with the mudstone or marl as the caprocks. These reservoir-caprock assemblages could be the potential hydrocarbon accumulation targets in the Ordovician, Tazhong area.
