| The Jizhong Depression is a Mesozoic-Cenozoic sedimentary depression within the Bohai Bay Basin that develops on the North China Platform.The Lower Paleozoic is part of the sedimentary caprock of the North China Platform,mainly developing carbonate rocks.Under the influence of multi-stage tectonic movements,an unconformity surface is formed on the top of the Lower Paleozoic,and is covered by the Carboniferous-Permian or Paleogene to form the buried hill reservoirs.In recent years,oil and gas have been discovered in several strata within the buried hill.This indicates that the Lower Paleozoic carbonate reservoirs are not only weathering crust karst reservoirs,but also other types of reservoirs.This study is aimed at the types,evolution,control factors and formation models of carbonate reservoirs,and it has important significance for buried hill reservoir prediction and oil and gas exploration in the Lower Paleozoic of the Jizhong Depression.The author started with the outcrops and wells of the Lower Paleozoic and established a stratigraphic correlation framework between the outcrop and the well.Based on the study of single well facies,seismic facies and well cross section facies,the paleogeographical pattern of the Lower Paleozoic in the Jizhong Depression is studied,and the petrology,stratigraphy,and paleogeography are clarified.Studies have shown that the Jizhong Depression was flat and broad in the Paleozoic.It mainly developed as a carbonate platform in the epicontinental sea.The carbonate platform facies include five subfacies,i.e.,the tidal flat,the restricted platform,the open platform,the oolitic bank,and the flat-pebble conglomerate bank.The initial transgression developed in the Fujunshan Age in the northeast,and formed a restricted platform and tidal flat.In the Mantou Age and Maozhuang Age,the scope of transgression gradually expanded,and the mudstone flat was developed.In the Xuzhuang Age,the tidal flat and the oolitic bank coexisted,and the sea level reached the highest during the Zhangxia Age.At the beginning of the Gushan Age,sea level fell and tidal flat reappeared.In the Changshan Age and Fengshan Age,the regression continued,and the flat-pebble conglomerate bank,dolostone flat,and limestone flat was developed.At the end of the Liangjiashan Age,the sea level was the lowest,and a regional unconformity was formed.Subsequently,the multi-stage of transgressive-regressive was developed at the beginning of Majiagou Age,and a palaeogeographic pattern with tidal flats and restricted platforms or open platforms was formed.There are three main types of reservoir space in the Lower Paleozoic: fractures,caverns(pore diameter ? 2 mm)and vugs(pore diameter < 2 mm).The fractures are mainly structural fractures,interlayer fractures,stylolites,and channels.The fracture is not only an important reservoir space,but also a channel connecting the caverns and vugs.The shape of the caverns is often irregular,and its long axis is either vertical or horizontal.In areas where the fracture dissolution is intense,the caverns and fractures form a large Cavernous-channels type of reservoir space combination.In the Carboniferous-Permiancovered strata with lithology mainly dolomite,a microfracture-vugs type of reservoir space combination is often developed.Studies on fractures and cements indicate that there are at least three phases of fracture in the Jizhong Depression.These three phases of fractures correspond to the three phases of rock rupture events of Indosinian movement,Yanshan movement,and Himalayan movement.Studies on the development and distribution of vugs,caverns,and channels show that there are the penecontemporaneous karst,the long-term unconformity karst,the short-term unconformity karst,and the burial karst.The long-term unconformity karst can further identify the angular unconformity karst,the parallel unconformity karst,and the faulted karst.The short-term unconformable karst includes the unconformity karst at the top of the Liangjiashan Formation and the unconformity karst at the top of the Fujunshan Formation.The burial karst can further identify the compaction released water karst and the hydrothermal karst.Lithology and sedimentary environments are the basis of karstification.The limemud limestones have a higher rate of karst cave development,while the fine-sized crystalline dolomites have more matrix pores.The sedimentary environment of dolomitic flat and dolomitic limestone flat is favorable facies for karst development.The penecontemporaneous dolomites formed in dolomitic flat have high matrix porosity,and the fractures and porosity in the thin interbed of dolomite and limestone formed in dolomitic limestone flat are relatively developed.The Caledonian karst paleotopography has an important influence on long-term unconformity karst.The Caledonian karst paleotopography was restored by the “residual thickness method” and “impression method”,and the data of the drilling break and mud loss of the well were superimposed,indicating that the karst slope has the most intense karstification.Karst depressions that do not accumulate water also have intense karstification.Long-term unconformity karst has zonation in vertical direction.Karst highlands,karst slopes,and karst depressions have karst zones of different depth range.The development of fractures has obvious promoting effect on karstification.The area where the fractures are densely developed and the porosity is also high.In the vicinity of the fault zone,there are more drilling breaks and mud losses.Under the regional tectonic stress,the fault network system is formed in the Jizhong Depression,and the Lower Paleozoic is cut into a series of fault blocks.A series of reservoirs are formed on the fault block.Hydrothermal dolomite reservoirs and hydrothermal karst reservoirs have been developed in the Lower Paleozoic due to the influence of tectonic hydrothermal fluids during the Himalayan.The hydrothermal process is related to the strike-slip faults or extensional faults formed by the Himalayan movement.Hydrothermal dolomitization and hydrothermal karstification were formed during the Paleogene magmatic activity.The hydrothermal reservoirs are distributed near the deep faults,and near the Paleogene intrusive rocks,especially in the negative flower structure.The lower strata of the short-term unconformity are less denuded.During the sea level fall slowly,dolomite associated with tidal flat is formed.Eogenetic karstification occurred during the exposure period,which forms a layered dissolution pore.After the sea level rise,the evaporative tidal flat deposits were generally developed in the whole area,and a large amount of high-magnesium brine was produced,making the general development of reflux seepage dolomitization under the unconformity.The development of dolomite provides a favorable basis for later karstification and reservoir formation.Based on the study of the characteristics,morphologies and controlling factors of each type of karst,the penecontemporaneous karst model,the long-term unconformity karst model,the short-term unconformity karst model,the compaction released water karst model,and the hydrothermal karst model have been established. |
PDF Full Text Request