Type, Genesis And Reservoir Of Dolomite In The Lower Cretaceous Langshan Formation In Cuoqin Basin From Tibet, China

The Cuoqin Basin in Tibet, which is a sedimentary basin from Cenozoic to Mesezoic and located between Bangongcuo-Nu river suture and Yaluzangbu river suture, tectonically belongs to Middle-Asia field of Tethys region and is mainly a Geji-Cuoqin back-arc basin, Bangongcuo-Nu river suture in north is involved.The basin,which size is 2.4×105㎞ 2,is the second largest hydrocarbon-generating sedimentary basin in Tibet.Systematically studying on genesis and oil-gas reservoir performance of dolomite located in the north-east ZhaGaNongHai county,we can draw following conclusions:(1) According to petrological characteristics, the dolomite of lower Cretaceous Langshan group which belongs to Cuoqin Basin Longgeer-Nanmulin stratigraphic section can be divided into two types: dolostone and calcareous dolostone, the dolostones almost comprise dolomites formed from dolomitization and recrystalization and their original textures had already been removed. The dolostones and calcareous dolostones is mainly composed of fine-crystalline dolomite, medium-crystalline dolomite and coarse-crystalline dolomite,the dolomite involved in dolostone are all relatively coarse.(2) Field petrology, skeletal grains, mirco-textures and cathodoluminescent lights of dolomite indicate that the dolomites in this study area all formed by replacement of calcite, but not original. Data of cathodoluminescent light, X-ray diffraction, inclusions,stable carbon and oxygen isotope, and salinity index Z which is calculated from data of stable carbon and oxygen isotope captured in dolomite further imply that they are mainly products of medium-deep burial and deep-burial compaction-squeeze dolomitization.(3) The data of porosity and permeability in carbonate from study area imply that the porosity of dolomite is 1.1～15.7% and the permeability of dolomite,which considerably varies,is 0.04～323×10^-3μm²,the minimum value of permeability from dolomite is 0.04×10^-3μm²,the maximum value of permeability from dolomite is 323×10^-3μm²,the average value of permeability from dolomite is 85.84×10^-3μm².Comparing with dolomite,the porosity of carbonate is similar,but the permeability of carbonate is 0.04～7.65×10^-3μm² and the average porosity of carbonate is 1.13×10^-3μm².So the dolomite is more suitable reservoir for oil deposit than carbonate and the assessment of dolomite reservoir range from good to better.Combining with examination of injected-pigmented thin sections,the data of dolomite porosity and permeability reveal that the dolomite reservoir mainly belongs to intermediate fracture-and-pore-type, low-porosity and medium-permeability, andâ… -â…¡-type oil reservoir in this area.(4) The results on analysis of relationships between dolostone type,dolomite shape,and different origin dolomites, and reservoir performance reveal that dolomite reservoir performance is relative to dolostone type,dolomite shape,and dolomite origin.Crystal dolostones are more suitable reservoir for oil deposit than calcareous dolomite, Of those crystal dolostones, the medium-coarse crystal dolomite is the most suitable reservoir for oil deposit;The reservoir performance of medium-deep burial dolostones are better than deep burial dolostones;The relationship between porosity and permeability of dolostones is irrelevant.