Hydrocarbon discovery in the Ordovician dolomite reservoirs in the Maigaiti slope, Southwest Depression of the Tarim Basin, and its exploration implications

Abstract

A high-yield gas flow is produced from the dolomite reservoirs of the Ordovician Penglaiba Fm in Well Luosi 2 in the Southwest Depression of the Tarim Basin, recording as the most important major oil and gas discovery in the Maigaiti slope in recent 20 years. In this paper, the success and failure of wells drilled into the Ordovician during the exploration in the Maigaiti slope and its periphery were analyzed to clarify the future exploration targets in this area. Then, the distribution characteristics of Ordovician limestone weathered crust and internal dolomite reservoirs were investigated, the relationship between structural traps (e.g. formation and evolution) and hydrocarbon accumulation was analyzed, and the main factors controlling the Ordovician hydrocarbon accumulation in this area were defined. Finally, the relationship between hydrocarbon distribution and Neoproterozoic rift system was discussed. Accordingly, some findings were obtained. First, the Ordovician dolomite reservoirs are more developed than limestone reservoirs. Limestone weathered crust reservoirs of Middle Ordovician Yingshan Fm (upper member) are mainly distributed in fault zones, and their reservoir space is mainly composed of high-angle unfilled fractures. Internal dolomite reservoirs of Lower Ordovician Yingshan Fm (lower member) and Penglaiba Fm are laterally and stably distributed and their reservoir space is mainly composed of small dissolved pores which present “strings of beads” reflection characteristics on seismic profiles. Second, the Ordovician hydrocarbon is mainly discovered in the fault zones and the lithologic traps consisting of Ordovician karst dolomite reservoirs at periclines are the main exploration targets in subsequent operations. Third, the distribution of Cambrian source rocks is the key factor restricting the success rate of Ordovician oil and gas exploration. Under the control of Neoproterozoic rift system in the SW Tarim depression, the middle section of this depression is favorable for oil and gas exploration, and it includes multiple hydrocarbon source–reservoir–caprock assemblages, such as Ordovician–Upper Cambrian dolomites and Carboniferous mudstones, and Lower Cambrian dolomite and Middle Cambrian gypsum salt rocks. Keywords: Tarim Basin, Southwest Depression, Maigaiti slope, Ordovician, Internal dolomite, Limestone weathering crust, Structural trap, Lithologic trap, Cambrian source rocks, Neoproterozoic rift system