关键词: 成岩—孔隙演化史, 成藏演化史, 碳酸盐岩, 微区地球化学分析, 栖霞组, 川西北地区

Abstract: Major breakthroughs have been achieved in the exploration of dolomite reservoir of the Middle Permian Qixia Formation in Sichuan Basin, which has become the focus for petroleum exploration and geological study. However, there are several problems such as thin reservoir thickness, strong heterogeneity, unclear reservoir distribution rule and main controlling factors for hydrocarbon accumulation. The core interval of 200 m is taken from seven typical outcrop sections and 10 wells in northwest Sichuan Basin, and core and thin section observations, as well as geochemical tests are conducted, including micro-area carbon and oxygen stable isotopes, micro-area rare earth elements, carbonate mineral laser in-situ U-Pb dating, cluster isotope temperature, and inclusion homogenization temperature, so as to constrain the diagenesis and hydrocarbon accumulation history of Qixia Formation dolomite reservoir. The following understanding is concluded in three aspects:(1) There are three types of dolomites of Qixia Formation in northwest Sichuan Basin, i.e., blocky dolomite, porphyritic dolomite and calcareous porphyritic dolomite, and six types of textural components are identified in various dolomites, namely, the limestone surrounding rock, anhedral crystal dolomite, euhedral crystal dolomite, fine crystal dolomite cement, saddle dolomite and bulk crystal calcite; (2) The anhedral and euhedral crystal dolomites were the products of dolomitization in the quasi-contemporaneous-shallow burying period (before the precipitation of fine crystal dolomite cement) superimposed by the recrystallization (dolomite accretion girdle, accompanied by the precipitation of saddle dolomite and bulk crystal calcite) in medium burial depth (2-3km) in the Indosinian period. Among them, the anhedral dolomite was generated by the metasomatism of grain beach limestone, and the reservoir space of blocky dolomite, composed of the anhedral dolomite, was dominated by dissolution pores; While the euhedral crystal dolomite was originated from the dolomitization of fine limestone filled in the meteoric karst system, and the reservoir space of porphyritic dolomite, composed of the anhedral and euhedral crystal dolomites,was dominated by intercrystalline pores of euhedral crystal dolomite. The reservoir space of both was essentially formed by the karstification of atmospheric fresh water in the early epigenetic period. Therefore, the paleomorphologic high areas are favorable exploration zones for dolomite reservoir; (3) The main hydrocarbon accumulation event of Qixia Formation in northwest Sichuan Basin occurred in the Indosinian period, represented by the cementation age of bulk crystal calcite (229 Ma±16 Ma and 229.3 Ma±3.4 Ma) with rich hydrocarbon inclusions,and experienced differential hydrocarbon accumulation in the Yanshanian- Himalayan, during which oil and gas reservoirs were destroyed in the thrust nappe zone, and gas reservoirs were re-formed in the thrust concealment zone and foreland sag zone. The understanding not only has important guiding significance for the exploration of dolomite reservoir of Qixia Formation in Sichuan Basin, but also supports to open up a new way for the study of reservoir development and hydrocarbon accumulation.