| In recent years, the focus of China’s oil and gas exploration direction is by land opposite marine, from clastic rocks to carbonate rocks, from the new generation to the Paleozoic, from shallow to deep, marine carbonate oil and gas exploration have made significant progress. With great buried depth and the temperature is greater than 140 oC, the development of dissolution pores, holes, fissure and other reservoir space in dolomite of Longwangmiao Formation of lower Cambrian in Sichuan basin reveals it is good dolomite reservoir, anti-order phenomenon that the porosity is still increasing with the increasing of depth was also found. It fully illustrate that deep dolomite can be high quality reservoir. So it is necessary to recognize development characters of dolomite reservoir, and exploring to establish the development mode of dolomite reservoir, based on which discussing the formation and development mechanism of marine carbonate reservoir deeply in study area, which is helpful for making true evaluation and prediction on distribution rules of these reservoirs.Based on the analysis of the domestic and overseas research situation of carbonate reservoir, a detailed analysis of the problems and extensive data collection and digestion, papers utilizes the theories of carbonate sedimentology, geology and geochemistry, reservoir geology, diagenesis etc as a guide, combines the existing achievement on Sichuan basin, takes advantage of the existing core data and drilling data etc, by the method of combine the macroscopic means and microcosmic means, combination of geology and logging, with the advanced analytical techniques means such as cathodoluminescence microscope, scanning electron microscopy, isotope analysis, x-ray diffraction analysis, inclusions, rare earth elements analysis, trace element analysis etc, conventional means combine with the new technologies, new methods, theoretical analysis combines with the experimental means, geology combines with geochemistry, form the research methods and ideas that multidisciplinary combines with professional knowledge, seize the two main factors(sedimentation and diagenesis) that affect the formation and development of reservoir, and emphasis on their correlation and dependence. We do a series of studies on the petrography of dolomite and the formation mechanism of dolomite and structure-hydrothermal dolomitization. Several main results and new knowledge as follow were obtained by the research.(1) Sedimentary systems of Longwangmiao Formation around the paleo-uplift are analyzed detailedly, which reveals restricted platform and evaporate platform are main sedimentary systems. Restricted platform with oolitic beach is widely develops on central region of Sichuan basin, while evaporate platform with gypsum and salt rock, sand and oolitic is widely develops on southeast of Sichuan basin, then establish their modes to interpret differences.(2) By classifying dolomite with composition-genesis, there are four types dolomites named dolomicrite, particle dolomite, grain dolomite and saddle dolomite, and the orders of them are gradually increased. This research summarizes the geochemistry of four types in depth on the basis of microstructures, which supply basic conditions for analysis of reservoir’s formation mechanism(3) The diagenesis types of dolomite in Longwangmiao Formation of lower Cambrian in study area are complex, such as dolomitization, cementation, compact and pressure solution, recrystallization, gypsum, dissolution, silicification, there is grading law that from penecontemporary dolomite â†' shallow burial dolomite â†'mid-depth burial dolomite â†'hydrothermal dolomite. It reflects the dolomitization environment was changed from meteoric water â†' shallow burial environment â†' mid-depth burial environment. And from diagenesis evolution degree, we made genetic evolution models that are fit for dolomitization in Longwangmiao Formation of lower Cambrian in study area.(4) Trace elements tell that dolomitisation of microlite dolomite is high saline sea water in open system, and particle dolomite with low content of Sr may be leached by meteoric water with low Sr. However, with the increase on content of Mn, grain dolomite formed under reduction condition, and formation of saddle dolomite is relate to hydrothermal fluid rich in Ba. Gross content of RE in saddle dolomite is higher than other three types of dolomites, which is further reveal hydrothermal fluid brings in other matters after formation of dolomite to make content of RE increase. The homogenization temperature reveals that grain dolomite is formed during high temperature. Because the homogenization temperature in saddle dolomite with structural fractures is higher than quartz in dissolution pores, holes and fissures, the formation of structural fractures is related to transformation effect of hydrothermal fluid made on dolomite.(5) The character of Carbon, Oxygen stable isotopes shows obvious differences between surrounding rock and fillers in miarolitic cavity, all of which tells there haven’t formed an balance between filling fluid and surrounding rock, and the outer fluid has low Carbon, Oxygen stable isotopes. But value of 87Sr/86 Sr is higher than synchronization seawater also show that Longwangmiao Formation transformed by fluid rich in Sr.(6) Over all evaluation, under background of multistage tectonism, syngenetic dolomitization grainstone of granular beach with grain dolomite dissolution of meteoric water in early time, which are with recrystallization in buried period and hydrothermal dissolution, dolomite reservoirs of Longwangmiao Formation in study area were formed after that became the high quality dolomite reservoir... |
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