| The oil and gas production of the world's carbonate reservoirs accounts for about 60% of the world's total oil and gas production,which shows the status and importance of the carbonate reservoirs.In recent years,a number of large-scale carbonate oil and gas fields have been discovered in the buried hills such as Ordovician of the Tarim basin platform basin and Ordovician of the eastern wing of the central paleo-uplift of the Ordos basin.Marine carbonate rocks dominated by the Paleozoic have become an important field of onshore oil and gas exploration in China.Most of these carbonate reservoirs are formed under the unconformity surface and are closely related to paleokarstification.Carbonate reservoirs have undergone a long process of karstification,resulting in a very complex reservoir law and strong heterogeneity,which makes exploration and development and reservoir prediction more difficult.Therefore,it is of great practical significance for oil and gas exploration in China to carry out in-depth research on paleokarst reservoirs in carbonate rocks and to grasp the distribution law and genetic mechanism of paleokarst.In this paper,the Ordovician carbonate rocks in the eastern part of Lunnan buried hill are taken as the research object,and the development law and genesis mechanism of ancient karst fissure-cave system are studied by using the method of combination analysis of paleokarst genesis.The research contents include the distribution and evolution characteristics of paleokarst landforms,the hydrogeological conditions of paleokarst,the development characteristics of paleokarst,the filling and evolution law of paleokarst,etc.The following procedures are as follows:(1)Guided by modern karst theory,an index system for the classification of paleokarst landform types is established,and the paleogeomorphic restoration and fine characterization are carried out by using the method of "genetic combination identification of paleokarst landforms".The study area is divided into four second-level geomorphic units and 12 third-level geomorphic units.(2)The palaeogeomorphological restoration technology is used to restore the palaeo-surface water system.Based on the recognition of geological and geophysical characteristics of karst fracture-cave system,the identification index system based on geomorphology,core,logging,seismic profile is established,the identification of palaeo-underground river system is carried out,and the pre-Carboniferous palaeohydrogeological conditions in the study area are systematically expounded.(3)Based on statistical analysis of basic data such as core,logging data,seismic profile and physical properties of fillings,the development law and distribution characteristics of ancient karst fracture-cave system in the study area are systematically studied:(1)The typical carbonate fracture-cave system in the study area is divided into four types: Underground river,karst cave,karst cave and karst fracture,and subdivided into eight sub-types,and the facies is established.The corresponding structural geological model;(2)Vertical division of the karst fissure-cave system in the study area into four zones: surface karst zone,vertical infiltration-cave zone,runoff-cave zone and subsurface-flow-cave zone,revealing the vertical difference of the development of ancient karst fissure-cave system;(3)The types and sources of filling materials were identified.Based on the systematic analysis of filling characteristics of fracture-cave system,the filling mode of typical carbonate fracture-cave system in humid environment was established.(4)The formation mechanism and evolution characteristics of the ancient karst fracture-cave system were systematically analyzed through stable isotope,inclusion,sporopollen and electron probe analysis and energy spectrum testing of fracture-cave fillings,combined with paleoclimatic conditions and karstification conditions,.The Ordovician weathering crust karst in the study area was mainly formed in the early Hercynian period,and the ancient karst fracture-cave system underwent three to four stages of karstification filling.(5)On the basis of the above research,the favorable blocks for karst reservoir development are forecasted from the perspective of palaeogeomorphology and palaeohydrodynamics.The prediction basis of stratum lithology,karst geomorphology characteristics,hydrodynamic conditions and vertical zoning of karst are systematically summarized.The favorable blocks of karst reservoir in the study area are forecasted.It is suggested that key exploration and development should be strengthened in the four blocks including Lunnan 11-Lungu 7-Lungu 4 well block(?). |
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