ANALYSIS OF ISOTOPES AND DIAGENESIS OF ORDOVICIAN DOLOMITE RESERVOIR IN THE WILLISTON BASIN

As a result of different stages of dolomitization,four types of dolomite rocks occurred in the Upper Ordovician Red River limestones of Williston Basin.Within the four types of dolomite rocks that are laminated micro-crystalline dolomite,burrow filled dolomite,matrix dolomite and saddle dolomite,only the matrix dolomite serves as reservoir with inter-crystalline pores.The composition of oxygen,carbon,and strontium isotopes of the four dolomites are obviously different from that of the isotopes of Late Ordovician marine water.Petrology and geochemistry study shows that dolomitization fluid flows of the former two dolomite rocks should come from Late Ordovician marine water,but today's their isotope composition reflect lateral rebuilding or modification.Reservoir dolomite rocks formed at a slightly higher temperature,and a rather later stage,and their fluid flows of dolomitization might come from marine water of Silurian or Cambrian-Middle Ordovician epoch or from thermal fluid in depth.The formation of saddle dolomites should be associated with deep thermal fluids.Occurrences of dolomitization along certain horizons are probably related to several thin layers of Kukersite shale that contains substantial organic materials in this area.