Mechanism For Formation Of Lacustrine Dolomites Of The Lower Xingouzui Formation In The Southern Part Of The Qianjiang Deprssion And Its Petroleum Geologic Implication

Half of the oil and gas resources in the world is concentrated in carbonate rocks, thus more and more attention is paid to researches on lacustrine carbonates in continental petroleum basins. Therefore, studies on lacustrine carbonate rocks with respect to deposition, reservoir characteristics, genesis and rules for oil and gas enrichment has an important implication for directing petroleum exploration. This paper is developed by using drilling, core, logging, seismic data as available as possible, as well as organic and inorganic geochemical analysis, on the basis of detailed investigation of previous relevant research results and under the guidance of basic theories involving carbonate sedimentology, reservoir geology and petroleum geology. In this paper, a systematic study is performed on the lacustrine dolomicrite rocks of Unit Ⅱ of the lower Xingouzui Formation in the Xingou Region of the southern Qianjiang Depression, regarding stratigraphy, reservoir deposition and their formation mechanism, as well as petroleum geological implication for their association with dark argillutite rocks.Through the SEM observations and X-ray diffraction analysis on rock samples from the study area,7 types of lithologies are identified over the study area, including dolomicrite, terrigenous clastic dolomicrite, micritic argillaceous dolomite and dolomitic mudstone, argillaceous gypsum, gypsiferous mudstone and siltstone rocks. For the accuracy of lithologic identification with logging data, and for the purpose of identification of main reservoirs, these seven types of lithologies are classified into dolomite, mudstone, argillaceous gypsum and siltstone rocks. On this basis, a logging model for identification of these lithologies is established by using the Fisher’s discriminant analysis approach, thus understanding the distribution and change of the dolomite reservoirs throughout the study area.Through analysis on facies markers of the lacustrine carbonate rocks, their electrical characteristics and well facies in the study area, the types of sedimentary facies, types of sedimentary facies are defined, including cloud flat microfacies and dolomitic sag microfacies. The cloud flat microfacies occurs in relatively high positions of the study area, where good hydrocarbon source-reservoir associations, as well as good oil and gas shows occur, providing favorable reservoirs for the study area. The dolomitic sag microfacies occurs in low parts of the study area, which is characterized by thick dolomite single layers, thin-bedded dark argillutite rocks sandwiched within dolomicrite rocks, showing a poor source-reservoir association, as well as poor oil and gas shows, incapable of serving as favorable reservoir microfacies in the study area. The comprehensive analysis on the sedimentary characteristics indicates that the development of the lacustrine carbonate rocks in the study area is mainly controlled by provenance, palaeogeomorphology and lake level fluctuation.Through SEM observation, cathodoluminescence, isotopic geochemistry analysis, X-ray diffraction for degree of order of the dolomite rocks, it is proposed that extensively-distributed, dark argillutite-interbedded dolomicrite rocks of Unit Ⅱ of the lower Xinggouzui Formation in the Qianjiang Depression were formed in a shallow burial, low-temperature diagenetic environment, which were sourced from common fluids, and formed due to the same mechanism. Their formations have experienced three phases: the syngenetic phase when dark argillutite and calcite were formed Penecontemporaneous seepage-reflux phase, when nonferrous dolomicrite rocks were formed; Diagenetic burial phase, when the dolomicrite rocks formed when the penecontemporaneous phase were adjusted and strengthened, resulting in large-scale development of the ferrodolomite over the study area. It was due to isomorphous replacement of Fe2+ in the formation water from partial Mg2+ in the dolomite rocks, rather than hydrothermal genesis.The sedimentary study on the reservoir rocks indicates that the dolomite reservoirs in the study area consist of terrigenous clastic dolomicrite, dolomicrite rocks and micritic argillaceous dolomite rocks, which are in a deteriorating order, with sedimentary facies as a main control. The micritic-structure dolomite reservoirs in the study area were formed in the early diagenetic phase, with well-preserved pores (dominated by secondary intergranular pores), due to less influence of diagenesis. The late seepage-reflux dolomitization also enlarged the pore spaces, and improved their petrophysical properties. In spite of fine grain sizes (dominated by micritic size), the dolomite reservoirs in the study area are characterized by moderate porosities and low permeabilities, capable of accumulating oil and gas.The petroleum geological implication for the dolomicrite rocks paragenetic with dark argillutite rocks indicates that the dark argillutite rocks interbedded with the dolomicrite rocks provided a good material base for hydrocarbon generation due to high organic abundances, in spite of low maturities in general. The environment for formation of the dark aregillutite rocks was favorable for formation of the dolomite rocks, resulting in thorough dolomitization throughout the study area, while dolomitization can increase pore volumes of the reservoirs, and improve their petrophysical properties. The oil-source correlation indicates that the crude oil of Unit Ⅱ of the lower Xingouzui Formation is undermature, which was mainly from the undermature source rocks within this unit. In the study area, source rocks, reservoirs and cap rocks were all formed at the same geological age, with the reservoirs interbedded with the source rocks, forming an association with reservoirs sourced from themselves.Finally, on the basis of the combination of the discovered oil layers, oil and gas shows, mechanism for dolomite formation, distribution of the sedimentary facies, effective thicknesses of the dolomite rocks, reservoir development and patterns for hydrocarbon accumulation, the distribution of the carbonate reservoirs in Unit Ⅱ of the lower Xingouzui Formation in the study area is predicted, Within the "C-shape" area consisting of the Xin areas 1 and 2, the favorable zones lie at relatively high structural positions, where the effective dolomite reservoirs of the three oil layers are superimposed in the study area. In addition, an oval area formed by tying wells Qiao 2, Xin 61 and Xi 2 is similar to the predicted favorable zones in seismic attributes, sedimentary setting and tectonic background, so the oval area may be an favorable exploration perspective area.