Carbonate Cap Rocks And Controlling Factors On Sealing Capacity

Carbonate cap rocks directly determine the hydrocarbon accumulation efficiency and preservation.Their scale and distribution affect the enrichment of oil and gas reservoirs.Analyzing the spatial and temporal distribution of carbonate cap rocks and their sealing capacity is of great significance for explorating carbonate reservoirs.This thesis presents a study of carbonate cap rocks of the Ordovician Yingshan Formation from the Tahe Oilfield and Keping outcrop analogues in the northern Tarim Basin,based on the analysis of samples from drill cores and thin sections,fullbore microscan imaging(FMI)and conventional well logs.Four qualitative patterns of cap rocks can be recognized from FMI analyses,with blocky-bearing,layered-bearing,porphyritic-bearing and linear-bearing images.Quantitative identification criteria can be established,with the higher value being deep-shallow dual lateral resistivity(RDRS)?1000 ?m and the lower one being natural gamma ray(GR)?15 API.Four lithological types can be recognized in the carbonate cap rock intervals,with mudstone being the most prevalent,followed by highly-cemented intraclastic or peloidal grainstone,dolomitic limestone and silty-fine crystalline dolomite.Five sets of cap rock intervals show a significant difference,and the thickness of individual cap rocks ranges from several meters to tens of meters.Carbonate cap rocks are well superimposed but laterally less continuous.Pore types of carbonate cap rocks are dominated by microfractures and intragranular pores,with minor contribution of intercrystalline and intracrystalline pores.The combination of mercury injection capillary pressure and nitrogen gas adsorption tests reveal six types of pore-throat structures can be identified.The fractal dimensions of macropores to micropores display a decreasing trend with decreasing pore-size diameters in wells,while an increasing trend can be observed from outcrop samples.These results all display that the sealing capacity of carbonate cap rocks increases with increasing fractal dimension.The cover coefficient,a parameter that allows characterizing the cap rock sealing performance,shows an increasing trend along with an increasing fractal dimension of the pore structure.The average cover coefficients of the six pore structure types show good correlation(either exponential or linear)with certain fractal dimensions.Capillary seals control the sealing capacity and the displacement pressure difference of effective sealing among the cap rock and underlying oil-bearing intervals should be 3 MPa or higher.The heterogeneity and anisotropy of macropores significantly control the sealing performances of carbonate cap rocks.The carbonate cap rocks with a good continuity were significantly controlled by the depositional settings and sequence stratigraphic framework and correspond to sediments of the middle-upper part of the transgressive system tracts(TST).They are likely to have been distributed in the intershoal sea,low-lying land of open platform and lagoon of restricted platform.Meanwhile,the carbonate cap rocks with a poor continuity or a thin thickness were considerably influenced with diagenetic alterations.Six diagenetic processes,including calcitre cementation,dissolution,mechanic compaction,chemical compaction,dolomitization and dedolomitization,affected the study cap rocks in wells and outcrops.The different diagenetic sequences experienced by samples from wells and outcrops resulted in a higher sealing performance of well samples,because they did not undergo diagenetic alterations associated with long-term sub-aerial exposure.The following conclusions have been found to enhance the sealing capacity of cap rocks:(i)grainstones have high sealing capacity when compaction preceded cementation,(ii)the increase of grain size in grainstones reveal a high proportion of intergranular pores which are likely to be cemented,(iii)highly-cemented grainstones present high sealing capacity when their cement content is equal or larger to 15% in volume,(iv)highly abundant stylolites filled with residual bitumen result in micropore occlusion and occlusion of the late hydrocarbon migration pathways,which can act as barriers for the upward migration of oil and gas.Four classes of cap rocks are defined here in accordance with their lithologies,thickness,pore structures,fractal dimensions and diagenetic alterations.Class I carbonate cap rocks,including highly-cemented intraclastic/peloidal grainstones and highy-compacted mudstone present the best sealing performance.