Interaction between source rock and evaporite: A case study of natural gas generation in the norther

ChinJ.Geochem.(2010)29:075- -081DOI: 10.1007/s11631-010-0075-0Interaction between source rock and evaporite: Acase study of natural gas generation in the north-ern Dongying DepressionJIN Qiang", WANG Juan', SONG Guoqi7, WANG Li', LIN Lamei', and BAI Shengpin'1 China University of Petroleum. Huangdao, Shandong 2566.6. China2 The Shengli Oilfield Co, SINOPEC, Dongying. Shandong 257001, China3 The Bohai Driling Engineering Co, PetroChina, Renqiu, Hebei 062550, China◆Corresponding author, E mail: jinyibo@hdpu.edu.cnReceived December 15, 2008; accepted January 12, 2009◎Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2010Abstract It is well known that oil generated from source rocks of saline lake facies is characteristic of low Pr/Phratio and high contents of gammaccrane. The authors found that gas generated from the same source rocks was highin benzene and cyclohexane contents and had light carbon isotope compositions. In this study, a series of thermalsimulations of gas generations on source rock with/without evaporites (such as carbonate, gypsum, and sodium chlo-rite, respectively), were conducted. It was found that the gypsum played an important role on the catalyzsis of gasgeneration in the thermal simulations. Compositions of the gaseous hydrocarbons generated from source rocks withevaporites are very similar to those of natural gases discovered from northern Dongying Depression. Meanwhile, inthe thermal simulations, it was found that the carbon isotopic compositions of gascous hydrocarbons generated fromsource rocks with evaporites are lighter than those of the gases from source rocks without evaporites. Therefore, it isconcluded that natural gases discovered from northem Dongying Depression are the product of interaction betweensource rocks and evaporites (especially gypsum) of the saline facies.Key words sal-lake facies; source rock; evaporite; interaction; gaseous hydrocarbon generationtype-III organic matter or source rocks of rather high1 Introductionmaturity (Dai Jinxin et al, 1992; Thopson, 1983; HuTilin et al., 1990; Tilley and Muehlenbachs, 2006).In recent years, a great amount of natural gas hasMeanwhile, methane in the gases found in northembeen produced from exploring wells drilled in sandyDongying Depression shows light carbon isotopicreservoirs at the depth of 4375- 4620 m in the Fourthcompositions (-47%0- -49%0) as compared with gasesMember of the Shahejie Formation (Esa) in northemfound in other areas, which indicates the maturity ofDongying Depression (Li Junliang, 2007), which in-the gases is much lower than the saline source rocksdicates that great discovery of deep oil/gas exploration(Linda et al., 2005). What is the reason for the gaseshas been made in the Shenli Oilfield. In northerproduced from the study area so different from otherDongying Depression, Es4 sandy reservoirs and sourcegases? The authors believed that Es4 evaporites playrocks are all saline deposits (Zhu Guangyou et al,an important role in the gas generation from the2004). Organic matter in the source rocks includessource rocks.types I and II, and the buried depths of the sourceIt is well known that oils generated from sourcerocks range from 4600 to 5500 m. The maturity isrocks deposited in a saline environment are character-corresponding to Ro 1.2%-1 .6%. However, composi-istic of even predominance of normal paraffin, phy-tions of the gases discovered in this area are high intane predominance and high gammacerane contents,cycloalkane and aromatic bydrocarbon contents (up toand the hupane series follows the order of C35 hopane25% and 38%， respectively) compared with gases>C34 hopane