Oil & Gas Geology ›› 2012, Vol. 33 ›› Issue (4): 599-606.doi: 10.11743/ogg20120415

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Late diagenetic dissolution of dolomites in the Feixianguan Formation, northeastern Sichuan Basin

Zhang Jun1, Cai Liulu2, Xiang Lei2, Cai Chunfang2   

  1. 1. SINOPEC Science and Technology Development, Beijing 100728, China;
    2. Key Lab of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
  • Received:2012-05-14 Revised:2012-07-11 Online:2012-08-28 Published:2012-09-11

Abstract:

Comprehensive analyses were performed on thin-section,carbon,oxygen and strontium isotopes and fluid inclusions of reservoirs in the Feixianguan Formation of northeastern Sichuan Basin.In the northeastern part of Kaijiang-Liangping shelf,intercrystalline pores resulting from dolomitization of calcite are absent in the high-quality reservoirs.In contrast,dolomite intergranular dissolution pores or intergranular enlarged dissolution pores are well developed in these reservoirs.Both two kinds of dissolution pores may coexist closely with calcites of light carbon isotope and occur in space between solid bitumen and dolomite crystals,thus they should have been produced by dolomite dissolution after solid bitumen precipitation.In the southern part of Kaijiang-Liangping shelf,dolostones are absent,thus pores of similar origin are also not developed.Under acidic conditions,dolomite may react with anhydrite or celestite,to produce MgSO4 in the northeastern part of Kaijiang-Liangping shelf.As the MgSO4 can be easily reduced by hydrocarbons to produce H2S, further promoting the dissolution of dolomite,anhydrite and celestite,the precipitation of giant crystal/poikilitic calcite,and the enlarging of secondary pores in local areas.This genetic mechanism can clearly explain the differences of reservoir physical properties between the northeastern and northwestern areas.The MgSO4 is likely to be the primary intermediate reactants of TSR.

Key words: dolomite dissolution, porosity evolution, diagenesis, TSR, Feixianguan Formation, northeastern Sichuan Basin

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