Distribution Patterns of Calcite Cement in the Turbidite Sandstones of the Shahejie Formation in the Dongying Depression,Eastern China

Calcite cements are volumetrically dominant among the most diagenetic constituents in turbidite sandstones of the Paleogene Shahejie Formation in the Dongying Depression.The results show carbonate cements mainly occur in three phases:Calcite I(pre-compaction),Calcite II(post-compaction)and ankerite in the turbidite sandstone.The isotopic composition of Calcite I and Calcite II range from−4.3‰to 4.4‰,−3.5‰to 4.3‰PDB,respectively,suggesting that the calcite cements are mainly derived from the interbedded shales.The early calcite cement precipitated from the mixing of lacustrine and meteoric waters at 38-69℃.The late-phase calcite precipitated from pore waters modified by water-rock interactions at 64-126℃.Two distribution patterns of calcite cements can be identified,depending on their position within the sandstone.The peripheral cementation of the sandstones is near the sand-shale contact.The development of cement is controlled by sandstone thickness and the properties of the interbedded shale.For scattered cementation,the distribution of the calcite cement is not controlled by the sand-shale contact.Calcite cement was preferentially distributed in coarse-grained sandstones,indicating that sandstones with high original permeability are the preferred pathway for the migration of early diagenetic fluids.

Selective dissolution of eodiagenesis cements and its impact on the quality evolution of reservoirs in the Xing＇anling Group,Suderte Oil Field, Hailar Basin, China

Reservoirs in the Xing＇anling Group in the Suderte Oil Field,Hailar Basin exhibit ultra-low to low permeability and high tuffaceous material content.This study comprehensively analyzed diagenesis and quality evolution of these low-permeability reservoirs using thin sections,SEM samples,rock physical properties,pore water data,as well as geochemical numerical simulations.Calcite and analcite are the two main types of cements precipitated in the eodiagenetic stage at shallow burial depths in the reservoirs.These two cements occupied significant primary intergranular pores and effectively retarded deep burial compaction.Petrography textures suggest selective dissolution of massive analcite and little dissolution of calcite in the mesodiagenetic stage.Chemical calculations utilizing the Geochemist＇s Workbench 9.0indicated that the equilibrium constant of the calcite leaching reaction is significantly smaller than that of the analcite leaching reaction,resulting in extensive dissolution of analcite rather than calcite in the geochemical system with both minerals present.Numerical simulations with constraints of kinetics and pore water chemistry demonstrated that the pore water in the Xing＇anling group is saturated with respect to calcite,but undersaturated with analcite,leading to dissolution of large amounts of analcite and no dissolution of calcite.Significant secondary intergranular pores have formed in analcite-cemented reservoirs from selective dissolution of analcite in the mesodiagenetic stage;the analcite dissolution formed preferential flow paths in the reservoirs,which promoted feldspar dissolution;and dissolution of such minerals led to the present reservoirs with medium porosity and low permeability.Calcite-cemented tight reservoirs have not experienced extensive dissolution of cements,so they exhibit ultra-low porosity and permeability.

TSR-Derived Authigenic Calcites in Triassic Dolomite,NE Sichuan Basin,China-A Case Study of Well HB-1 and Well L-2

It has been proven that thermochemical sulfate reduction （TSR） took place extensively in the Lower Triassic carbonate reservoirs in Northeast （NE） Sichuan （四川） basin. We have carried out analyses on bulk rock compositions and isotope ratios together with petrography and fluid inclusions to assess the impact of TSR on diagenetic process of Triassic dolomites. In this article, TSR-related burial diagenesis is characterized by precipitation of calcite cement with negative 613C values and high ho- mogenization temperature. The light carbon isotopic compositions of this phase indicate that carbon incorporated in this cement was partly derived from oxidation of hydrocarbon. The high homogeniza- tion temperatures indicate that the thermochemical reduction of sulfates has been taking place in the deep part of NE Sichuan basin. Additional evidence supporting this interpretation is the high Sr values of this calcite cement. Moreover, the calcites have a 6180 of -8.51%o to -2.79%0 PDB and are interpreted to have precipitated from high salinity fluids with 6180 of ＋5%0 to ＋13%o SMOW. Under cathodolumi- nescence, these calcite cements appear dark brown or black, and both Mg concentrations and Mn/Sr ratios are low. It is therefore indicated that seawater was the principal agent of precipitation fluids. Finally, it should be noted that although H2S and CO2 increased as TSR continued, porosity has been ultimately destroyed by calcite cementation.

Calcite Cements in Burrows and Their Influence on Reservoir Property of the Donghe Sandstone, Tarim Basin, China

Abundant burrows of Skolithos linearis, Palaeophycus tubularis, and Taenidium barretti are preserved in the Upper Devonian Donghe sandstone of Tarim basin, China. They are commonly highlighted in core by the color of the burrow fill in contrast to the surrounding matrix and have dif-ferent textures and fabrics from the matrix. There are three kinds of calcite cements in burrows, microcrystalline-crystalline calcite cement, mosaic calcite cement, and ferroan calcite cement. Microcrystalline-crystalline calcite cement is widely distributed in the lower part of the Donghe sand-stone, but it is rare in burrows filling. It displays a globule structure under burial cementation and sel-dom replaces the quartz grains in shallow burial depth stage. Mosaiccalcite cement is widespread in the Donghe sansdtone reservoir of North Tarim basin. It shows chrysoidine, orange and bright orange lu-minescence, and intensely replaces the quartz grains, forming in early diagenetic and shallow burial depth stage. Ferroan calcite is asymmetrically distributed in the Donghe sandstone reservoir of Central Tarim basin. It fills the remanent pores in the shape of mosaic and replaces the quartz grains, matrix, and early calcite cement, forming in late diagenetic and deep burial depth stage. The burrows filled with white calcite cements have low oil saturation or may be oil-stained. In contrast, there is high oil saturation in the sandstone reservoir where the bioturbation is sparse or not present. With increased bioturbation, the porosity, permeability, and oil saturation decrease; thus, bioturbation intensity and reservoir property appear to be negatively correlated.