Element Geochemistry Evaluation Of Marine Source Rock With High Maturity

Marine source rock is currently one of the important oil and gas exploration areas.It has thick sediment, develop several sets of main source rocks. However,the quality and quantity of organic matter of these hydrocarbon source rock changed a lot, suffered long-term thermal evolution and transformation of violent tectonic movement.the traditional method of hydrocarbon source rock evaluation of the application in the region face many difficulties, especially chloroform Bitumen "A", hydrogen index and so the evolution of indicators by the effects of high error will appear. This article is for the highly evolved marine source rocks and the proposed new method for evaluating source rock.In this paper the lower Cambrian of Yangba Profile in Nanjiang, Sichuan has been studied. Taking the basic principle of petroleum geochemistry and basing on field records,the Geochemical testing of trace elements as well as carbon and oxygen isotope is used to illustrate the mainly facts of developing marine source rock, proposing a set of method of element geochemical for evaluating source rock. The main results and conclusions are as follows:1. According to the observation and sampling of fieldwork, the characteristics of organic matter, major elements, trace elements and carbon isotopes are analyzed in the Lower Cambrian of Yangba profile in Nanjiang,Sichuan.Qiongzhusi group which the average TOC is greater than 1%, is the high-quality hydrocarbon.Other groups have a low content of organic carbon,belong to non-source rock. Organic matter type is sapropelic, high maturity each group.In addition, the testing results show that the major elements constant of black carbonaceous shale, especially Al, Ba, Ca, P, Cu, Zn, K, Na and S which related to biological growth is higher in Qiongzhusi group, trace elements,such as V, Cr, Ni, U and Mo, also has higher content in the bottom of Qiongzhusi group, the upper part, V, U, Mo content of the elements are significantly reduced, REE diagram also shows a more enriched in light rare earth elements and heavy rare earth element characteristics of a slight loss.Xiannvdong group developed oolitic limestone and calcarenite. The results reflect the content of major elements,such as Al, Ba, Fe, P, Ti, K, Na and S, are decreased heavily,while the contents of trace elements V, Cr, Co, Ni, U, Mo, Th and Pb are reduced, rare earth model curves keep slightly level.Yanwangbian group mainly developed silty mudstone, the content of major elements have a tendency to increase in the bottom, while still low in the upper. The trends of trace elements is similar to major element.In addition to Pb, there is declining. Rare earth model curves slightly level.Shilongdong group developed dolomite. Generally, the content of major elements and trace elements are lower, the mode curve of the rare earth element showed that a more enriched in light rare earth elements.In addition,the carbon isotope have two cycles.2. Based on the theory of element track, the key control element of marine source rock formation in region of interest is researched by the measurement of field profile, combining with the result of interior analysis test about total organic carbon and element. We start with the paleo-productivity, fossil oxygen facies, burial efficiency and hydrothermal activity to denote the ocean paleo-productivity by Baxs, reflect the oxido reduction environment at the time the OM sediment by V/Cr, Ni/Co, U/Th and the rare-earth element including La/Yb andδCe, indicate the burial efficiency of organic carbon by Mo, instruct the submarine hydrothermal solution action's influence to the OM accumulation by Ba/Sr.The analysis result indicate that the lower of Qiongzhusi formation in lower Cambrian has a high paleo-productivity, the OM sediment in an anaerobic environment, the burial quantity of organic carbon is high, and the geobiology condition is advantage to develop superior source rock. The paleo-productivity of Xiannvdong formation and Yanwangbian formation is low. The sedimentary environment deviates to be poor oxygen-rich oxygen facies which is disadvantage to the conservation of OM, and the quantity of organic carbon is little. The burial efficiency of the bottom of Shilongdong formation is relatively low, while the paleo-productivity is big and the sedimentary environment is anoxic. Analyzing from this point, more paleo-productivity, anoxic environment and high efficient burial rate provide good environment for the formation and development of superior source rock. Otherwise, the submarine hydrothermal fluid also affects the accumulation of OM and controls the development of superior source rock.3. The element geochemistry can reflect the environment of source rock, affect the concentration and has a closely correlation to the organic carbon content which reflects the abundance of OM. Under the direction of this theory, the elements which are correlated to TOC, inorganic carbon isotope and organic carbon isotope are analyzed to build three fitting equations with the TOC, inorganic carbon isotope and organic carbon isotope as variable and optimized element content as argument. The nutrient elements affecting the growth of biology such as Baxs,Mnxs,Fexs, the sea water salinity element Srxs,and the elements affecting the conservation condition of OM such as Uxs,Vxs,Nixs,Znxs are chosen to evaluate the abundance and deposition, conversation environment of OM. At the same time, factor analysis is conducted to the elements including Moxs,Uxs,Fexs,Mnxs,Vxs,Nixs,Znxs,Srxs and Baxs to obtain a compositive factor calculation equation combined by multi-element which includes total information about the change of trace element of the OM in sedimentary rock in the process from formation to deposition and bury. It reflects the changes of the OM sedimentary environment and the rules of OM distribution and accumulation. It can be used to evaluate the superior and inferior of the source rock.The analysis result indicates that the lower of Qiongzhusi formation in lower Cambrian has a higher compositive factor score which reflects that the sediment quantity is more, the reservation condition is advantageous and superior source rock can develop. The compositive factor score of top Qiongzhusi formation, Xiangnvdong formation and low Yanwangbian formation is between 0 and 1 which shows ordinary source rock may develop. Whereas the compositive factor score of the middle and top of Yanwangbian formation and Shilongdong formation is smaller than 0 which shows the OM quantity is little and the bury condition is so bad that it's unfavorable for the source rock development.4. It is a new clue to evaluate highly evolved marine source rock by the characteristic of element geochemistry. It has some unity to the result of traditional source rock evaluation, but some differences still exist. This method can be used as an important addition to the source rock evaluation of highly evolved area in southern China.