Characteristics And Evaluation Of The Weathering Karst Reservoir At Top Leikopo Formation In Central Sichuan Region

Under the guidance of multi-disciplinary theories and methods, such as sedimentary petrology, reservoir geology, karstology, logging geology, geomorphology and so on, based on take full advantage of data from regional geology, field outcrop, drilling, logging, oil testing, core and the previous studies, this paper systematically researched the weathering crust karst of Leikoupo Formation in central Sichuan region, and established the karst structure patterns, studied the basic characteristics of the weathering karst reservoir, controlling factors of its development and distribution, and the distribution characteristics of litho-logy, the palaeogeomorphology and fractures. The paper also evaluated and analyzed the distribution of the weathering karst reservoir by means of comprehensive evaluation method with controlling factors weight of the grey relationship analysis. The main conclusions are as follows:1. Based on the field outcrop, the field karst structure profile was established. Field weathering karst has obviously zonation in radial direction, which can be divided into four bands that are surface karst belt, seepage flow karst belt and underflow karst belt and deep slow flowing belt. The surface karst belt is relatively rare. According to the structure and composition of the breccias, the underground weathering karst can be divided into three bands, seepage flow karst belt and underflow karst belt and deep slow flowing belt. The seepage flow karst belt and underflow karst belt have distinctive characteristics that can be identified easily.2. Based on the field outcrop, drilling, logging and analytical material, the recognition marks for ancient karst of Leikoupo Formation in central Sichuan region were built up, including stratigraphic features, mineral-rock characteristic, the product of diagenesis, geochemical characteristics of stable isotope and cathodoluminescence and the logging features.3. Using the method of "from point to surface", based on the analysis of the typical blocks’reservoir, the reservoir of the top of Leikoupo Formation in the study area was analyzed. Reservoirs in central Sichuan region have low porosity-(special) low permeability. The physical property of Longgang is the best, followed by Moxi area and Yuanba zone. There is a certain relationship between lithology and physical property. The physical property of dolostone is better than that of limestone. The physical property of granular limestone is the best in limestone. Granular dolomite and powder crystal dolomite are the best in the dolomite, which are the best reservoir rock of Leikoupo Formation. Reservoir spaces are mainly intercrystalline pore, intergranular pores, intercrystalline dissolved pore, intergranular dissolved pore. Karst caves are not developed, which are mainly distributed in Longgang zone. Small Karst caves rarely disappeared in Yuanba area.4. In the aspects of the characteristics of weathering crust karst reservoir, paleoclimate, palaeostructure and lithology, several typical areas were compared. It is concluded that many objective factors of the top of Leikoupo Formation in the central Sichuan region, such as the ancient climate and paleo-hydrological conditions, favorable lithology and its thickness and dissolution time and so on, are worse than those of carbonate karst reservoirs in China. They do not have the subjective conditions to develop karst to be large-scale, and the growth degree of the weathering karst reservoir is limited as a whole. The local growth degree of weathering is good5. Regional weathering karst reservoirs are mainly controlled by the lithology (layer), followed by palaeogeomorphology, and impacted by fractures locally. Granular dolomite and powder crystal dolomite are the most conductive lithology for karst, and dissolved high flat and dissolved flat in karst slope are the favorable ancient palaeogeomorphology for karst.6. The paper selected the "impression method" and "Residual thickness correction method" to restore the ancient landscape. The ancient landforms of the superface of Leikoupo Formation of the Longgang, Moxi, yuanba and Jieshichang zone were restored by "impression method" that used the superface of Leikoupo Formation and subface of NO.3of Xujiahe Formation as datum level. The ancient landforms of the superface of Leikoupo Formation of the central Sichuan were restored by "Residual thickness correction method’" which took the superface of Leikoupo Formation and NO.2of Jialingjiang Formation as datum plane. The palaeogeomorphology of the central Sichuan are the slope whose southwest is steep and northeast relatively flat. The palaeogeomorphology in Longgang. Moxi, Yuanba and Jieshichang are all low-lying from east to west, three-level palaeogeomorphologic units consists of dissolved high flat, dissolved flat, gentle slope and solution pan.7. The primary fracture within the area is structural fracture with the following features: having little width, usually less than lmm; the length of fracture mainly ranging from5cm to10cm, being medium length fracture, having low filling degree, unfilled or half-filled mostly.8. Based on the fracture prediction by the finite element method in typical zones, the distribution of fracture in the central Sichuan Basin was predicted. The result is that the first level of fracture development area of the central Sichuan are major in the north-east structural group of central part and the north-west structural group of northeast part of central Sichuan, such as southern Suining-Yuechi, northern Tongnan-Hechuan, northern Xichong-Guangan and Yilong-Yingshan-Quxian. They are the zone with high development degree fault and structure.9. The weathering crust karst reservoir on the top of t Leikoupo Formation in the central Sichuan region was evaluated by the "comprehensive evaluation method of reservoir controlling factors". Firstly, the weight coefficient of lithology, palaeogeomorphology and fracture are0.38,0.32and0.30, respectively that were calculated by Gray Relationship Analysis. This method is applied to central Sichuan, the weight coefficient are0.42,0.32and0.26, respectively, which matched with the three relations from geological analysis. Secondly, the criteria were pointed out for reservoir evaluation by "Q-mode cluster analysis". Then, the reservoir was evaluated comprehensively. The first level of reservoir development region is mainly distributed along Longgang-Yuanba and Chongxi-Nanchong. That of the second level is distributed in Longgang-Yuanba, Chongxi-Nanchong, well Gaoke-1-well Anping-1-well Moxi-3-well Longnv-1in Moxi zone and well Wangjia-1-well Moxi7-well Hechuan12in Tongnan area.