Characteristics of the Reef Flat Facies Reservior in the Puguang Gasfield and the Origin of Dolomitization
Received date: 2012-02-07
Revised date: 2013-05-04
Online published: 2013-10-10
The Triassic Feixianguan Formation reservoir in the Puguang Gasfield is a typical phase-control reservoir and is composed of highly dolomitized oolitic shoal.It is dominated by residual oolitic dolomite,intraclast and micritic dolomites.The reservoir spaces are various and the dominant types are intergranular pores and internal pores (include intracrystalline pores).Few fractures are developed in local areas.Different types of reservoir space have very different physical properties,pore structure parameters and sensitivity characteristics.According to further analysis,it is proposed that the causes of different types of reservoir spaces are related to lithofacies and dolomitization.At the bottom of the beach,it developed transgressive micrite limestone (dolomites),including particles like oolitics and sand cuttings which are micrite structure.At the low location of the beach,it developed inner hole oolitic dolomite.The oolitics are usually composed of microcrystalline dolomite,which has low dolomitization.At the middle and high location of the beach,it developed residual coarse-grained intracrystalline pore residual oolitic dolomite and the oolitic is coarse-grained structure.At the top of the beach it mainly developed crystal dolomite,including anhydrite.The physical properties of reservoir gradually become poor,and it is easy to form fractures.The lithologic changes of the oolitic beach body reflect that as the water becomes shallow,grains of the dolomites increase constantly,and the degree of the dolomization strengthens constantly.In addition,the oxygen and carbon isotopic values demonstrate that δ18Ο of oolitic grain-refining dolomite is negative with an average of -5.037‰(PDB).The average of δ12C is 1.596‰ (PDB).The analysis of trace element show that Sr and Na are at a low level.The average content of SrO and Na2O is 0.018 72% and 0.013 12%,respectively.The average content of FeO is 0.057 64%.The x-ray diffraction analysis of dolomite order degree is low,i.e.,0.62 on average and below 0.9 generally.The CaCO3 molar fraction in Mg/Ca is high,with an average of 51.14% and generally more than 50%.There′s nearly no fluid inclusion in the rock section,and some has little single-phase fluids inclusion.This shows that the dolomite of the oolitic beach body of Puguang was formed under the condition of the meteoric fresh water,oxidation,low temperature,and near the surface,which implies the dorag dolomitization causes.
JIANG Nan,FAN Ling-xiao，LIU Hui，LIU Zhi-yuan,MEI Bing . Characteristics of the Reef Flat Facies Reservior in the Puguang Gasfield and the Origin of Dolomitization[J]. Natural Gas Geoscience, 2013 , 24(5) : 923 -930 . DOI: 10.11764/j.issn.1672-1926.2013.05.923
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