The Development Mechanism And Controlling Factors Of The Early Cambrian Longwangmian Formation Reservoir Of Reef And Bank Facies In The Sichuan Basin And Its Peripheral Regions

With the growth of energy demand,depletion of conventional oil and gas resources and progress the exploration technological,unconventional oil and gas resources and deep resources gradually are the main exploration targets in worldwide especially in china.The successful exploration of the lower cambrian Longwangmian Formation in the central Sichuan Basin reveals enormous exploration potential of deep carbonate reservoir and provides more detailed geological basis and theoretical guidance for deep exploration.The reef and bank facies have been regarded as the most important hydrocarbon reservoirs in the cambrian Longwangmian Formation in the Sichuan Basin,but the development mechanism and controlling factors of these reservoirs remain particularly controversial.The development mechanism and controlling factors of reef and bank facies has been studied based on the new datas,theories and methods of petrographic and geochemical characteristics.The deposition,dolomitization,dissolution,reservoir characteristics,diagenesis sequence and porosity evolution of reef and bank facies have been detailed analysis and discussion.The main achievements and new understandings as follows:?1?Stratigraphic sequence and sedimentary systems of the Longwangmiao Formation are analyzed detailedly.Two third-order sequences are identified in most areas of the Sichuan Basin and three third-order sequences are identified in local area of peripheral regions.Rimmed carbonate platform has been regarded as main sedimentary system.The restricted platform and platform margin with reef-bank facies developed widely in the Sichuan Basin and its peripheral regions.Based on core and outcrop data,reef-bank types of the Longwangmian Formation are identified,including intraclastic oolite and bioclastic banks,stromatolite and thrombolite.According to geological,seismic and logs datas,recognition marks of reef-bank facies are summarized.On the basis of the above studies,the sequence and stratigraphic frameworks are established and the development characteristics of reef-banks in frameworks are analyzed.Finally,the distribution and evolution law of reef-bank facies are intensively described in system tracts.?2?According to petrographic and sedimentological features of the different composition and fabric in reef-grain shoals,the sedimentary environment and pattern of reef-bank facies are analyzed.Grain banks locating at the core of reef-grain shoals mainly developed in the high energy and shallow water environments.The stromatolites with alternating bright and dark laminae mainly formed in the semiarid,mesohaline and weak hydrodynamic conditions and developed on leeward side of reef-grain shoals.The thrombolite with a clotted texture formed in moderate to turbulent hydrodynamic conditions on windward side of the shoals.The stromatolite and thrombolite commonly occurred in the wing of reef-grain shoals.Based on a comprehensive research,the development of reef-grain shoals is controlled by paleostructure?palaeogeomorphology?,fluctuation of sea level?sequence development process?and microbe activities.?3?Five dolomite types have been identified in this study:dolomicrite?Dol-1?,microbial doloboundstone?Dol-2?,sucrosic dolomite?Dol-3?,medium to coarse crystalline dolomite?Dol-4?and saddle dolomite?Dol-5?.The?¹³C and?¹⁸O values,⁸⁷Sr/⁸⁶Sr ratios and rare earth element?REE?pattern of Dol-1,Dol-2(except the?¹³C values)and Dol-3 are approximately consistent with original seawater values,suggesting that the dolomitization fluids were mediated by seawater under shallow burial conditions.The high calculated dolomitization temperature?25.66°C?and Z values?125.2?of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation.The calculated dolomitization temperature?24.20°C?and Z value?121.8?of Dol-2 and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater.The more negative?¹³C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation.The dolomitization temperature?30.55°C?and Z value?125?of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater.However,Dol-3 is commonly distributed in reef and shoal facies?high porosity and permeability?throughout the Qingxudong Formation.Moreover,special marks of sabkha dolomitization?e.g.,evaporite layers,dry crack and birds-eye structures?were not found.The abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization.The geochemical data(except the?¹⁸O values)of Dol-4 are approximately consistent with fabric-retentive dolomites,suggesting that the Dol-4 had been inherited from precursor carbonates.The negative?¹⁸O values of Dol-4 revealed a further burial environment.The high LREE/HREE ratios,positive europium anomalies and negative?¹⁸O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions.Thus,Dol-1,Dol-2 and Dol-3 are likely attributed to sabkha,organogenic?microbial?and seepage-reflux dolomitization models during early-stage diagenesis,respectively.However,Dol-4and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis.?4?Five stages of dissolution have been identified in the Longwangmian Formation:syn-sedimentary dissolution?karst?,pene-sedimentary karst,buried karst,hydrothermal karst and epidiagenetic karst.Firstly,the syn-sedimentary and pene-sedimentary karsts occurred in eogenetic stage.The syn-sedimentary karst has been attributed to meteoric freshwater corrosion and the pene-sedimentary karst?including shoreland and island dissolution?was mainly mediated by mixed water.Subsequently,the buried and hydrothermal karsts formed in mesogenetic stage.The buried karst was mainly related to organic matter dissolution and the hydrothermal karst was attributed to thermochemical sulfate reduction?TSR?.The syn-sedimentary and pene-sedimentary karsts controlled by reef-bank facies developed conduit-intergranular dissolved pores that are the most importance karst types in the Longwangmian Formation reef-bank facies.The other karsts played some roles for further redistribution or creation of porosity.?5?Through comparative analysis of reservoir space characteristics,physical characteristics,diagenesis and porosity evolution of the Longwangmiao Formation in the central Sichuan basin,the southeast Sichuan basin and the west of Hubei Province and east of Guizhou Province.The controlling factors of the Longwangmian Formation reservoir of reef and bank facies in the Sichuan basin and its peripheral regions are summarized as follows.?1?Deposition of reef-grain shoals occurring in ancient landform highland is the fundamental control factor for reservoir of reef and bank facies.?2?Dissolution?especially syn-sedimentary and pene-sedimentary karsts?is the principal control factor for the reservoir space formation.?3?Dolomitizing?especially early dolomitization?can effectively prevent the post cement formation and has been regarded as the key control factor for preserve of reservoir space.?4?Thermochemical sulfate reduction?TSR?related to hydrothermal process is a double-edged sword for porosity development in the Longwangmian Formation reef and bank reservoirs,but the destructive effect of TSR played a more active role.?5?Reticulate tectonic fractures created by multi-stage tectonic movements greatly improved the permeability of the Longwangmian Formation reef-bank reservoirs.In conclusion,early dissolution and dolomitization associated fracturing are the fundamental control factors of the Longwangmian Formation reef and bank reservoirs.Extensive and contiguous reef-bank shoal deposits in the ancient landform highland are the best locations for oil-gas exploration.This study is especially useful for further understanding deeply buried reef-bank reservoirs formation and development,and deep hydrocarbon exploration in this basin and elsewhere worldwide.