Evolution of Depositional Architecture of the Ordovician Carbonate Platform in the Tarim Basin and Its Response to Basin Processes

Key words: 
• carbonate platform /
• sequence architecture /
• depositional evolution /
• Ordvician /
• Tarim Basin

Abstract: The Tarim Basin contains extensive carbonate platform deposits of the Ordovician System, with more than three thousand kilometers of rimmed carbonate platform margins. Comprehensive analysis of seismic, borehole and outcrop data reveals that the Ordovician carbonate system in the basin can be divided into four composite sequences defined by major unconformities. Ten sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into eight third-order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early-rifted Manjiaer aulacogen, the carbonate platform margins of the Lower and Middle Ordovician that prograded eastward in an arcuate belt extending generally north-south across the northern part of the basin. The development of the Tazhong uplift resulted in an extensive unconformity between the Middle and the Upper Ordovician in the south-central basin, and subsequently constrained the formation of an isolate carbonate platform, with their margins controlled by marginal thrust-fault belts of the paleo-uplift during the Late Ordovician. In the northern basin, the Late Ordovician carbonate platform margin developed around the marginal slope of the Tabei paleouplift. The transgressive regressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. The reef shoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.