| The practice of oil and gas exploration shows that lots of marine oil and gas fields at home and abroad are closely related to gypsum-salt rocks.In recent years,the cream formation of carbonate hydrocarbon generation,reservoir formation and accumulation of influence is becoming a hot spot.However,the genesis of gypsum salt rock has always been controversial,especially in the complex tectonic-sedimentary background,the genetic model of the assemblage of gypsum-carbonate coexistence and its influence on the pre-salt oil and gas accumulation are relatively weak,which seriously restricts the exploration and development of the oil and gas reservoirs related to gypsum salt rock.The Cambrian system in Sichuan basin has a complex tectonic-sedimentary background because of its multiple tectonic movements,With the continuous breakthrough of deep-ultra-deep subsalt oil and gas exploration,Anyue Longwangmiao formation super large gas field was discovered,but other areas with good oil and gas display failed to make a big breakthrough.In order to further understand the influence of gypsum salt rocks on oil and gas accumulation,this study takes the Cambrian gypsum salt rocks in the Sichuan Basin as the research object,under the guidance of relevant theories such as carbonate sedimentology and petroleum geology,the rock types and distribution rules,sedimentary environments,lithofacies paleogeography,geochemistry and reservoir characteristics of Cambrian and internal gypsum salt rocks were comprehensively analyzed by using a variety of data,and the following understandings were obtained.(1)The rock types,characteristics and distribution of the gypsum salt rock were identified,and the variation trend of the distribution of the gypsum salt rock was revealed.The results of systematic statistics and comprehensive analysis of more than 60 field and drilling profiles of Cambrian in the Sichuan basin show that the Cambrian salt rocks in the Sichuan basin are mainly composed of pasty dolomites and gypsum rocks,which are characterized by wide distribution,many intercalations,large thickness variation of single layer,more gypsum rocks and less halite.The gypsum salt rock layer is mainly developed in the Middle and Lower Cambrian,the Longwangmiao Stage(the Qingxudong formation,the Longwangmiao formation,the Shilongdong formation),in the upper part of which the Longwangmiao stage gypsum salt rock is developed.Gypsum salt rocks of the Middle Cambrian are mainly distributed in the Gaotai formation and the Qinjiamiao formation,with the largest thickness in the southeast Sichuan-northwest Guizhou and northeast Chongqing-west Hubei.Layers contained the gypsum salt rocks shows a gradually distribution trend from the Longwangmiao formation(the Qingxudong formation)of the southeast Sichuan-northwest Guizhou to the Qinjiamiao formation(the Gaotai formation)of northeast Chongqing-west Hubei.The Chongqing-Luzhou and the Lichuan-Pengshui areas are the two depocenters of gypsum salt rocks in the lower Cambrian.On the whole,the distribution shows the characteristics of "mountain shape" which have the transition from southwest to northeast and "ring belt shape" in trending near NS direction.The middle Cambrian gypsum salt rocks is more widely distributed,and the depocenter is mainly in the Wanzhou-Lichuan area,with the distribution characteristics of "bull's-eye" in trending near NS direction.The depocenter has trend from the southeast Sichuan-the northwest Guizhou of lower Cambrian area to the northeast Chongqing-the west Hubei of middle Cambrian area.(2)The sedimentary paleosalinity and paleotemperature and lithofacies paleogeographic characteristics of the gypsum salt rocks were analyzed,and the forming environment and main controlling factors of Cambrian gypsum salt rocks in the Sichuan basin were identified.Palaeogeographic pattern from the Longwangmiao period to the middle Cambrian is as follows:land at west,a stage as main body,a lake at the stage,basins on east and north edge respectively.From the late early Cambrian,large-scale regression occurred in the Sichuan basin,and the sedimentary environment became relatively closed,with strong evaporation in the warm subtropical climate.The paleo-temperature and paleo-salinity tended to rise first and then decrease(the average temperature was 21.30?,the Z value was 124.29,and the average Sr/Ba ratio was 4.75),reaching the highest point from the late Longwangmiao period to the middle Cambrian,which was the most favorable stage for the development of the gypsum salt rock.When the sea level is high,the shallow shoals in the carbonate platform are relatively developed.The intershoal lagoon,which is mainly composed of pasty salt deposits,is developed in the landform depression between the shoals.When the sea level decreases,the shoals are connected into walls,which can form multiple intershoal lagoons of different sizes.The gypsum salt rocks are mainly formed in this kind of closed and semi-closed intershoal lagoon environment,the sedimentation of which is mainly controlled by tectonic movement,palaeogeomorphology and palaeoclimate.(3)A new genetic model of "multiple lagoons" of the gypsum-carbonate coexistence with evaporative genesis was established.There are many lagoons of different sizes in the Cambrian period in the Sichuan basin.Based on the difference of gypsum-carbonate coexistence and the factors such as hydrodynamic force,water sealing degree and water depth,the "multiple lagoons"salt-salt rock formation model with evaporative origin,such as still water lagoons,disturbed lagoons and mixed tidal depressions was established.The still water lagoon is a relatively deep and restricted depression in the upper part of the supra-intertidal zone.Evaporation of surface sea water results in the replacement of high-salinity sea water with low-salinity sea water at the bottom.In addition,the backflow of other high-salinity brine in the environment results in the formation of association of gypsum-salt rocks with dolomite rocks or shale rocks.The disturbed lagoons and intertidal depressions are located in the shallow water environment of the relatively low-lying part of the supra-tidal zone and the exposed area of the supra-tidal zone,where the water is turbulent.Lithofacies assemblage mainly consists of rhythmically interbedded association of gypsum-salt rocks and dolomite rocks and association of dolomite or mudstone(sand)rocks with gypsum salt rocks.The thickness of single layer of gypsum salt rock is generally thin.The exposed construct is a common construct type.The Cambrian gypsum-salt rocks are mainly formed in the still water lagoon in the southeastern Sichuan-the western Hubei and the eastern Chongqing,followed in the disturbance lagoon in the eastern Sichuan,and the mixed tidal depression in the southwestern Sichuan.(4)The influence of the gypsum salt rock on the pore evolution of the diagenetic stages of the dolomite reservoir was clarified.The Cambrian gypsum salt rocks in Sichuan basin catalyzed the hydrocarbon generation of the lower Cambrian source rocks,thus accelerating the thermal evolution of organic matter.In the syngenetic period,the dissolution rate of the dolomites containing gypsum salt is higher than that of the dolomites without gypsum salt.Karst paste mold hole is the main reservoir space of dolomite reservoir,which is symbiotic with gypsum salt rock.During the shallow burial period,organic matter provided the material basis for the dissolution of gypsum and metasomatism of carbonate rocks,so as to improve the physical properties of the reservoir.The dissolution in the supergene karst stage further improves the reservoir physical properties.During the middle and deep burial stage,TSR reaction enhanced the dissolution.The dissolution holes in the burial stage are inheritable.The gypsum salt rocks of the Longwangmiao formation contributed the most to the porosity of dolomite reservoirs in the syngenetic and penecontemporaneous stages.(5)The comprehensive evaluation parameters and evaluation criteria for the reservoir control of the gypsum-carbonate coexistence are proposed.The thickness and lithologic assemblage characteristics of gypsum salt rock,tectonic(fault)activity intensity,overlying strata preservation,hydrocarbon source rock development characteristics,reservoir characteristics,and(pure)argillaceous cover layer development were selected as the comprehensive evaluation parameters for the control and accumulation of gypsum-carbonate coexistence.The evaluation results show that the three most favorable zones are the south central Sichuan,the west southeast Sichuan and the east and west Chongqing.The Leshan-Suijiang area on the eastern edge of the basin is a favorable zone.The remaining four zones are all unfavorable zones. |
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