| Affected by the Caledonian movement,the North China platform was lifted and experienced weathering and erosion for 140 Ma.Karst reservoirs are widely developed on the Ordovician weathering crust in Ordos Basin.The exploration and development of the karst reservoirs are mainly concentrated in Jingbian area and Daniudi area in the central part of the basin.The Hangjinqi area,which is close to the Yimeng uplift in the northern part of the basin and characterized by high intensity of karstification and low crustal-opening strata,is relatively low at the exploration degree.In recent years,cavernous gas reservoirs have been drilled in the limestone strata of Ma4 Member in Hangjinqi area,which have shown good exploration potential.At present,the ordovician weathering crust in Hangjinqi area lacks systematic geological comprehensive research.the distribution of strata,sedimentary facies and paleogeomorphology are not very clear,and the laws of reservoirs development and gas-water distribution are uncertain.Based on the previous research results,using the core data,logging data and related seismic methods,the stratigraphic characteristics,sedimentary evolution,ancient geomorphological distribution,reservoir development characteristics,and reservoir accumulation patterns of Ordovician karst reservoirs were systematically studied,and favorable zones are predicted.the following research results were obtained:(1)The weathering crust of the Hangjinqi area exposes the Ma4 Member,Ma3 Member,Mesoproterozoic and Archean basements from south to north.The Ma3 Member was deposited in the marginal facies of the tidal(upper tidal and upper-intertidal zone),which mainly composed of dolomite,gypsum dolomite,mudstone and sandstone.The seawater deepened during the deposition of the Ma4 Member,which was an open subtidal sedimentary environment.The lithology of Ma4 Member is represented by bioclastic micrite limestone,which can be compared with the Ma4 Member of the basin.The upper part of the Ma 4 Member was eroded during the Caledonian tectonic period.(2)Based on the analysis of the thickness of Ma4 Member and the Taiyuanformation overlying the Ordovician unconformity,combined with the seismic facies characteristics above the Ordovician unconformity,five tertiary paleogeomorphic units are divided,and the paleogeomorphology of the pre-carboniferous period is comprehensively characterized.The study area was generally high at the northwest and low at the southeast in the Pre-Carboniferous period,with 5 gullies and 4 ridges developed in the east-west direction.(3)The karst reservoirs include pore-type and cave-type reservoirs in the limestone of Ma4 Member.The pore-type reservoirs are developed within 6m of the surface of Ma4 Member and is generally filled with mud.The cave-type reservoirs are distributed inside Ma4 Member.The development of caves is controlled by three stages of water table,with three layers of caves superimposed,60-80 m,85-105 m and about 120 m below the datum plane.In the paleogeomorphic valley,the upper caves have been eroded and encountered at a low rate.(4)The internal structure of the cave can be divided into the bottom collapse face,the cave sediment filling face,the cave unfilled face and the cave roof face from bottom to top.Seismic coherence attribute analysis shows that the cave appears curved and braided on the plane,and closed sinkholes are developed in some areas.Seismic coherence attribute analysis shows that the cave appears curved and braided on the plane,and closed sinkholes are developed in some areas.(5)The pore-type reservoirs in the surface of Ma4 Member develops up-dip pointed lithologic gas reservoirs on the basis of mudstone shielding,and natural gas is distributed at present high structural area.Inside Ma4 Member,cave-type gas reservoirs present various isolated units.According to the principle of cave development,mudstone sealing,and away from the fault zone,five favorable areas are predicted on the basis of the cave's plane distribution law. |
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