Features Of Key Unconformities And Their Controls On Reservoir Distribution Of Ordovician In Tazhong Area, Tarim Basin

Based on the theories of sequence stratigraphy, seismic stratigraphy,sedimentology, structural geology, and reservoir geology and on the basis of theformer achievements of sedimentary facies, the paper systematically studies thevertical and lateral features of the two key unconformities (T₇⁴ and T₇⁰) of Ordovicianfor the first time. The features include the overall background of development, thedrilling features, the seismic features, and the qualitative denudation features by usinglithofacies sequence. Besides, the paper indicates the significance of the two keyunconformities for the oil and gas exploration.Take the denudation and palaeogeomorphology as the masterstrokes, the authordid some research on the restoration of denuded thickness and palaeogeomorphologyof the two key unconformities (T₇⁴ and T₇⁰). To quantificationally study the featuresof denudation, the author adopted the methods of vitrinite reflectance (Ro) and theextrapolation of formation tendency. The research of T₇⁴ shows that the maximumdenuded thickness lies around the well Badong 2, and its maximum is over 400m;Thebiggish denuded thickness basically lies in Ka 1 block and on the west of the area ofthe well Badong 2;In the rest of the Tazhong area, the denuded thickness is graduallythinner and thinner from the middle to the southern and northern sides. The researchof T₇⁰ shows that the maximum deduded thickness is from 1000m to 1200m, muchmore than that of T₇⁴, and lies around the well Tazhong 1, Tazhong 3 and Tazhong 5.Besides, the author quantificationally studied the features of palaeogeomorphology ofT₇⁴ and T₇⁰. The research of T₇⁴ shows that the uplands of Tazhong area lies in thelines of the well Tazhong 13-Tazhong 18-Tazhong 9 and the well Tazhong10-Tazhong 11. Its palaeogeomorphologic features are: higher in the west and lowerin the east, higher in the middle and lower in the around, smooth terrain landform, andnot very developed structures. The research of T₇⁰ shows that the uplands hadtransferred to the lines of the well Tazhong 1-Tazhong 5-Tazhong 38 and theGuchengxu uplift in the east of Tazhong area. Its palaeogeomorphologic features are:higher in the east and lower in the west, higher in the middle and lower in the around,steep terrain landform, and extremely devoloped strctures. The quantificational studyof the denuded and palaeogeomorphologic features of the unconformities providesstrong criterions for the favorable zones predication of the karst reservoirs.Finally, by means of integrated analysis of sedimentary facies, and the features ofdevelopment, denudation and palaeogeomorphology of the key unconformities, theauthor did some comprehensive forecast of the karst reservoirs controlled by theunconformities T74 and T70, and as a result, the author points out the most favorableand the potential zones of the karst reservoirs of Ordovician. To test the rationality ofthe predication, the author chooses some wells in different zones to contrast thedeveloped features of the karst reservoirs. Through the analysis, the predication iscoincident with the exploration practice.