Formation Mechanism Of The Lower Permian Dolomites In Western Sichuan Basin

Diagenesis and diagenetic environment of carbonate is one of the most importantcore fields of sedimentary geology and petroleum geology. Among these diageneticresearches, the dolomitization and the formation mechanism of dolomite attract manygeologists in long time. Dolomites of the Lower Permian Qixia Formation were studiedthree decades ago in Sichuan Basin and its periphery areas. Recent explorations suggestthat the dolomites of western Sichuan Basin have good reservoir quality and could beseen as the new exploration target. Dolomites mainly occur in2nd member of QixiaFormation, and have the thickness of meters to tens of meters. Based on availableexploration information, these dolomites have excellent hydrocarbon explorationpotential. This dissertation, based on mineralogical, petrologic and geochemicalinformation of different carbonate phases in Qixia Formation, studies the characters,origin, evolution and control factors of multistage diagenetic fluids, and also researchesthe relationship between diagenetic fluids and dolomite precipitation and limestonedissolution. Combining the information of magmatic activities, tectonic movement,paleoclimate, sea-level change, sequence stratigraphy, burial history and othercarbonate diagenetic events, the formation mechanism of carbonate reservoir in LowerPermian Qixia Formation of western Sichuan Basin is studied, the basic theoretic issueof dolomitization is researched, and the carbonate diagenetic theory and model(especially the dolomitization model) in Lower Permian Qixia Formation of westernSichuan Basin is established. Current research provides the essential theoretic conceptfor carbonate reservoir exploration in Lower Permian Qixia Formaton of westernSichuan Basin, and also provides the valuable case study for dolomitization insedimentary basin.Fabric-preserved dolomites are absent in Lower Permian Qixia Formation of western Sichuan Basin. Almost all rocks are crystalline dolomite rocks. And the puredolomite rock is also absent in current study, suggesting that the duration ofdolomitization was short, the magnesium ion in strata was limited, and thedolomitization was incomplete. The crystal sizes of dolomites are large, also indicatingthat the concentration of magnesium ion was relatively low and the number ofnucleation site was poor. Dolomites mainly have the crystal size of fine toextremely-coarse. And the dolomite fabrics include planar dolomite rhombs, planarsubhedral to euhedral crystals, non-planar anhedral crystals, non-planar saddle crystals,dolomite overgrowth and dolomite crystals with fragment and dissolved features. Thesefabrics indicate the elevated temperature in dolomitization or dolomite precipitation.Authigenic non-carbonate minerals like illite, fluorite, quartz, fluorapatite and baritewidely occur in pores and vugs of dolomites, especially in the dissolved dolomite vugs.These authigenic minerals could be the indicators of external hydrothermal fluids indolomitization.All fluid inclusions in different carbonate fabrics in Qixia Formation have highhomogenization temperatures (Th). The average Thvalue of dolomite fabrics is135.5℃,higher than calcite fabrics (109.5℃). In different dolomite fabrics, the non-planarsaddle dolomites have the highest average Thvalue of151℃, the non-planar anhedraldolomites also have high average Thvalue of140℃. Different planar fabrics haverelatively low Thvalues. The average Thvalues of dolomite rhombs, euhedral crystalsand subhedral crystals are89℃,103℃and109℃, respectively. The dolomiteovergrowths have higher Thvalues than its host crystals, suggesting the dolomitegrowth were accompanied with progressively elevated temperatures. Although thesaddle dolomites have the highest Thvalues, the later formed calcites in saddle dolomitedissolved pores have the lower Thvalues. The average Thvalue is78℃. The mineralprecipitation order was contrary to geothermal gradient of progressive burial,suggesting the temperatures of dolomite precipitation were not caused by geothermal,and the temperatures of diagenetic environment in Qixia Formation had retrogradationafter the precipitation of saddle dolomites. The temperature decrease was more than70℃. Such scale of temperature fall would lead to the fragmentation of saddledolomites.By combining the research results of homogenization temperatures, oxygenisotopic compositions of minerals and oxygen isotopic compositions of fluids, thecharacters of dolomitizing fluid in the Lower Permian Qixia Formation of westernSichuan Basin are identified by inversion analysis method. All dolomitizing fluids have the higher salinities than sea water, that is to say the different dolomite fabrics wereproduced by or precipitated from saline water or hypersaline water with elevatedtemperatures. The fluids that precipitated saddle dolomite have the oxygen isotopevalue of+8to+12‰SMOW, and could be seen as hydrothermal fluids with highsalinity. The fluids that produced other dolomite fabrics have the oxygen isotope valueof0to+8‰SMOW. Compared with the dolomites that precipitated from normal seawater, these fluids also could be seen as hydrothermal fluids. Most of the calcites wereprecipitated from the fluids of normal marine salinities, the oxygen isotope value ofprecipitating fluids are from-5to+1‰SMOW.Based on the fabrics and compositions of different carbonate and non-carbonatephases, the corresponding element and stable isotope geochemistry information, themicrothermometry of fluid inclusions, and the burial and thermal histories, currentresearch concludes that the formation mechanism of mainstream dolomite fabrics in theLower Permian Qixia Formation was related to the thermal event caused by Emeishanbasalt/mantle plume. The dolomitization was an incomplete and turn-round process.The dolomitizing fluid was the mixture of marine-derived water and limited externalion/fluid, and had higher salinity and higher temperature than sea water. However, theorigin of magnesium was still the marine-derived water in strata. The thermal eventcaused by Emeishan basalt/mantle plume plays a significant role in dolomitization ofQixia Formation. The elevated temperature overcomes the dynamic obstacle indolomite precipitation, and contributes to dolomite precipitation under the aqueousenvironment of low magnesium concentration or low Mg/Ca ratio. The short durationof thermal event and the shortage of magnesium in strata lead to incompletedolomitization. Therefore, the dolomitic limestone and limy dolomites were producedas major phases. Once the thermal event terminated, the formation temperaturedeclined rapidly. In addition the depletion of magnesium, the dynamic obstacle ofdolomitization was reconstructed, leading to the termination of dolomitization.Considering the retrograde dissolution of lower temperature and the depletion ofmagnesium in diagenetic fluid, the dolomite would be fragmented and dissolved,especially the saddle dolomite precipitated in elevated temperature.