Characteristics Of Hydrothermal Dolomite Of Upper Sinian Dengying Formation In Southeast Sichuan Basin

This paper focuses on hydrothermal dolomite of Upper Sinian Dengying Formation in southeast Sichuan basin. After a detailed observation of regional tectonic evolution, lithology and sedimentary features of Dengying dolomite in study area, His conclusion from petrological and geochemical aspects are that some hydrothermal alteration occurred in Dengying dolomite. At the same time, this paper sums up reservoir petrography and geochemical charecteristics of hydrothermal dolomite, and points out main diagenetic features in reservoir formation. Finally, reservoir formation and evolution of hydrothermal dolomite is presented. Through these processes, the paper finally gets some significant results below:After formation of the basement due to Jingning Movement, the sedimentary and tectonic evolution of study area can be divided into five stages:①Nanhua glacial period(Nh);②passive continental margin and uplift/depression formation stage(Z-S);③Emei Taphrogeny period(D-P);④foreland evolution stage(T-K1);⑤folding and uplifting stage(K2-Q). During this evolution history, two important structurally controlled hydrothermal events took place—Xingkai Taphrogeny and Emei Taphtogeny, which formed upward conduits for deep hydrothermal fluids.The Upper Yangtze region was a restricted platform during Late Sinian. The study area is comprised dolomite deposited in tidal flat and lagoon environment. This dolomite is interpreted to be mostly primary in origin, little of it could be produced by dolomitization. Crystalline dolomite from fine to coarse crystalline and algal dolomite are the main types of the rocks. Algal dolomite is critical for Dengying Formation. It's including algal bindstone, algal stratifera, algal stromatolite, algal thrombolite and seldom algal oncolite. In addition, snowflake dolomite, silicified dolomite and hydrothermal dolomite formed during diagenesis are also important.Hydrothermal dolomite reservoir was generated when Dengying dolomite was subjected to hydrothermal alteration during late diagenesis. Hydrothermal alteration may produce a range of dissolution and precipitation features, and may have variable impact on Dengying reservoir characteristics. For instance, it is common to see saddle dolomite, a critical indicator of hydrothermal setting, and many distinct rock fabrics including zebra textured dolomite, subhorizontal sheet vugs lined or infilled by saddle dolomite dilational, floating clast dolomite breccia cemented by massive saddle dolomite; Thermal discoloration occours on wall rock of hydrothermal dolomite. Because of hydrothermal alteration, Dengying dolomite appears to be mottled; There are also MVT deposits and barite-fluorite ores, which are suggested to have genetic link to hydrothermal dolomite.Hydrothermal dolomite in Lin One well has abundant solution pores, giving good log response. Especially, within 46m from the disconformity between Cambrian and Sinian, where hydrothermal fluid were particularly active. This may indicate that the unconformity was a advantage fluid passage. The fact that hydrothermal alteration enhances porosity of Dengying dolomite is demonstrated by porosity and permeability experimental data of Lin One well. These data show matrix porosity, don't include vug porosity which is the main contributor to reservoir. Abundant bitumen is found in pores of hydrothermal dolomite implicating that hydrothermal dolomite once was good hydrocarbon reservoir. Hydrothermal dolomite porosity includes solution pores, vugs and intercrystalline pores.In comparison to wall rock(original Dengying dolomite), dolomites in veins or/and vugs show lowerδ13C andδ18O values, higher 87Sr/86Sr values.δ13C,δ18O and 87Sr/86Sr values imply that Dengying dolomite was subjected to diagenetic alteration, and that the diagenetic fluid was from outside source. 87Sr/86Sr values indicate this diagenetic fluid may be hydrothermal fluid flowing up via active fault, from Liantuo Formation of Nanhua System below or basement. This conclusion is supported by homogenization fluid-inclusion temperature data and petrographical characteristics.By core and thin section examination and geochemical analysis, it's concluded that the following main diagenetic events in hydrothermal dolomite reservoir of study area are: dolomitization, compaction and pressure solution, silicification, cementation, dissolution, recrystallization, hydrothermal alteration, tectonic fracturing and so on. Among them, dissolution, hydrothermal alteration, recrystallization and tectonic fracturing have important control on reservoir porosity. Because early formed porosity is infilled during late diagenesis, the most significant dissolution is supposed to be the hydrothermal dissolution and deep-burial dissolution. Hydrothermal alteration ocourred during the Early Permian, and could be reflection of Emei Taphrogeny in southeast Sichuan basin.