Origin Of Dolomite And Dolomite Reservoir Development In Middle Ordovician M₅⁵ Sub-Member Platform Carbonates In The Daniudi Area, Northern Ordos Basin, Western China

As the exploration in Daniudi gas field turn into the mature stage, the exploration of the weathering crust karst reservoirs in Ordovician have made great progress in recent years, and it shows that the gas reservoirs has better ability to give high and stable yields. However, it is imperative to provide resource guarantee to important exploration region of reserve replacement in Daniudi gas field. The successful explorated precedent of M55 sub-member dolomite reservoirs shows that it has great exploration potential. Therefore, the paper focused on the actual requirement of the carbonate gas reservoirs in Ordos Basin, to interpret and document the origin of dolomite and the dolomite reservoir, and prediction for the dolomite reservoir, based on sedimentology, core samples and the physical properties of log and geochemical method.According to the particularity exploration of dolomite tight gas and oil in Majiagou Formation in Ordovician, many scholars regard the layer of the M55 sub-member as the aquiclude of the Karst, and called as "black belt" due to the black dense muddy limestone. In recent years, the exploration of oil and gas got important breakthrough and the lithology of the gas reservoir is the dolomite, however, the dolomite reservoirs is heterogeneous which leads to great difficulties to the dolomite reservoir prediction. Therefore, the paper discusses the characteristic of dolomite petrography and the distribution of dolomite, and a comprehensive summary of the dolomite petrography characteristics include the following formations: ?the dolomite are recognized in the M55 sub-member based on the crystal size, plane, boundary shape (non-planar or planar) and contacting relation:(1) the microcrystalline (<40?m), sub-to euhedral inclusion-rich dolomite; (2) finely crystalline (40-200?m), euhedral to subhedral dolomite crystals. According to the features of the lithology, summarized the superimposed relationship between limestone (packstone, wackstone and micrite limestone) and kinds of dolomite, which suggest that the volume of dolomite is diminishing downward; ? the gypsum-bearing microcrystalline dolomite commonly developed in the top of M55 sub-member strata;? the microcrystalline dolomite is primarily represented by laminated and gypsum-bearing muddy dolomite, and the fine dolomite is primarily present as discrete, thickly bedded bodies distributed unevenly;? there is a close spatial relationship between the abundance of the microcrystalline and finely crystalline dolomite and the microcrystalline dolomite developed on the top of finely crystalline dolomite of substantial thickness. Study on the sedimentary sequence cycle, and show that the lithologic variation is controlled by the third sequence and the high frequency sequence, which fought out that the dolomite is developed in the slow regressive period and the limestone is developed in the rapid transgression period.Using reflux model of penesaline sea water to interpret the origin of the M55 sub-member dolomite, based on the carbonate petrography and geochemical characteristics, and synthetical elaborated the relationships of reflux dolomitization and the high frequency sequence on gentle slope in Ordos Basin. Stratigraphic, petrographic and geochemical data indicate that the microcrystalline dolomite formed in near-surface, low-temperature, slightly evaporated sea water, and the dolomitizing fluids may have been driven by density differences and the elevation head, and the microcrystalline dolomite have high 80 values, 87Sr/86Sr ratios than the M55 sub-member limestone, but slightly lower ?18O values than the M55 sub-member marine dolomite, which indicate that the microcrystalline dolomite undergo the recrystallization in the penecontemporaneous period. The fine dolomite has lower ?18O values (-8.5 to -6.7‰) and Sr2+ concentrations and slightly higher Na+ concentrations,87Sr/86Sr ratios (0.709188 to 0.709485), and Fe2+ and Mn2+ concentrations than the microcrystalline dolomite, these suggesting that the fine dolomite precipitated from modified brines and dolomitic fluids in pore water and that the fine dolomite developed in slightly higher temperature conditions as a result of shallow burial. Whereby summarized the dolomitization genetic model in the process of transgression and regression cycle. In addition, generalized the evolution of diagenetic sequences and the diagenetic evolution pattern and make a conclusion:favorable diagenesis for the dolomite reservoir development include dissolution, shallow burial dolomitization and fracturing, and the damage effect to the development of reservoir development include cementation, compaction, pressure solution, dedolomitization and the formation of authigenic mineral filling.The porosity generation of the dolomite reservoir was studied based on the analysis of dolomite genesis and diagenesis. Studies have found that the development of the dolomite intercrystalline pores have close relationship with dolomite volume and structure. A high content of dolomite and the high degree of shape, the porosity of dolomite is well developed. The microcrystalline dolomite developed little dissolved pores and intercrystalline pores due to the recrystallization, the intercrystalline pores of the fine dolomite is the result of the shallow burial dolomitization, and set up a porosity genetic model of the shallow burial dolomite with reflux model. In the model, evidences suggest that in the updip direction of the platform, the crystallization of dolomitization fluid is progressing rapidly and dolomitization fluid supply is adequate, which lead to the over-dolomitization, and developed a set of microcrystalline, to anhedral to subhedral and relatively dense dolomite. The dolomitization fluids in shallow burial stage derived from reflux model of the overlying strata, and the dolomite fluid supply rate slows down, the subhedral to euhedral fine dolomite possess abundant intercrystalline pores, following that, it is too lack dolomiting fluids to provide and lead to partial dolomitization, and lime dolomite commonly have low porosity. This porosity development model has closely relationship to the reflux dolomitization model and sequence cycles. Studies have found that the karstification in supergene stage block off the pores in the dolomite reservoirs and the dedlomitization occurs in the top and bottom of M55 sub-member which damaged most dolomite pores. And on this basis the paper summarizes the pore evolution of the dolomite reservoir.The papers summarize the characteristics of M55 sub-member reservoir, and thought that the reservoir space mainly include intercrystalline pores and intercrystalline solution pores, based on the full account the factors affecting reservoir development and distribution, using the CT scanning technology more-refined depicted the pores and throats of the dolomite, and interpret the origin of low porosity and low permeability of dolomite reservoir, in addition, this model gives out an effective method which study the tight gas pore dolomite reservoir. Study have found that there is a good corresponding relation between the low porosity and low permeability of dolomite reservoir and the resistivity and neutron curve, on the basis of these discussions, marking the comprehensive evaluation to the dolomite reservoir and analysis results in combination with petroleum geological conditions, and the paper predicted the favorable reservoir exploration area.