A Study On Interaction Of Source Rock And Oil With Evaporates In Saline-Lake Facies

A large number of natural gas and oil are discovered in saline-lake facies. The deposition of organic matter in saline is different from that in freshwater lake. In the oil-gas generation, there may be the interaction of source rock and oil with evaporates in saline-lakes. Natural gases from saline maybe have sepecial geochemical characteristics. These are related to oil-gas genese in saline lake. Furthermore, oil-gas generation has affected the direction of oil field exploration and resources evaluation in saline-lake.Basing on the identification of relationship against oil-gas reservoirs and source rocks in saline, this paper conducts a series of thermal simulations about organic matter with /without evaporates in low and high evolution phase. The simulation results show that carbonate, gypsum, and sodium chlorite intensive catalysize on organic matter degradation and gases product has certain geochemical characteristics. Furthermore, combinating with the organic matter evolution in Minfeng sag, gas generation modle is established in saline-lakes facies.It is worth noting that the natural gas in Minfeng area is of special geochemical characteristics such as the index of methylcyclohexane is about 38%, the index of benzene is about 26%,the methane carbon isotope composition is from -48.95 to -44.66‰. These characteristics don’t match with the maturity of orgain matter（Ro=1.1-1.6%）. Hence, the gas generation causes controversy.For the low evolution kerogen-degraded gas, different salts、pyrolysis temperature and thermostatic time affect the yield of gas generation in varying degrees. Simulated temperature plays an important role in the gas generation. Adding carbonate or gypsum to the source rock sample, the number of catalysizing hydrocarbon would raise 34% and 28% spectively. It is found that generating gas is with some geochemical characteristics such as relative wet hydrocarbon composition and high benzene content.It is discovered that source rock-gupsum and oil-sodium chlorate has the largest gas production compared with other salts in high evolution. On the analysis of each C_5-8 hydrocarbon and light hydrocarbon parameters evolutation, it is found that gypsum and sodium chlorite could accelerate the process of hydrocarbon generation from organic matter. The catalysizing essence is that the degraduation reaction of is advanced to the following higher evolution phase （thermal temperature）.The hydrocarbon composition of kerogen-generated gas and oil-cracked one are of relative drying gases, high content of cycloalkane and aromatic in high evolution phase. The gas index of C₅ cyclane/C₅ paraffin from kerogen with evaporates is more than 1.6 and the parameter ration of oil-cracked gas is lower than 1.3. Otherwise, this ration is more than 1 in the the low evolution phase of oil cracking.Generating gas from source rock or oil sample with evaporates is more light carbon isotope composition compared with those from the simulated samples without evaporites. The carbon isotope of kerogen-degraded gas with/without evaporates is from -33.9‰to -34.8‰, the one of oil-cracked gas with/without evaporates ranges from -42.4‰to -44.4‰. In addition, analogying with the the carbon isotope of gas product from thermaled samples of organic matter single degraded or degraded with other salts, the one of organic matter with sodium is more light 1-2‰.The paper selects parameters such as iC4/nC4、the index of alkand-aromatic、the index of cycloalkaneⅠandⅢ. Comparing with the above index, it is found that gas product from organic matter degraded with gypsum or sodium chlorite is more similar to natural gase from Minfeng sag. The similarity coefficient of source rock samples with evaporates and natural gases is much close than the one of oil samples with evaporates. Comprehensive the analysis of the gas product, carbonate isotope compostion and geological setting, it is believed that natural gas found in Minfeng sag is generated from the interaction of source rocks-gypsum, followed from oil-halitethe cracking. Furthermore, only the interaction of organic matter and gypsum-halitethe could explain the problem why the gases geochemical characteristics don’t match with the evolution of organic matter in Minfeng area.Based on the parallel first order reaction kinetics model, it is found that the decreasement of the activation energy is the mechanism to increase the hydrocarbon number from orgaic matter pyrolysised. Evaporates influence gas generation and change the activation energy distribution of hydrocarbon. In addition, the dynamic model of hydrocarbon generation could make applicaton with this area evolution and establish gas generation model of organic with gypsum or halitethe in saline basin. After doing quantitative calculation, it is obtained that gypsum or halitethe could effect the hydrocarbon generation threshold and evolution history of organic matter（source rocks and ancient reservoir）.