Petroleum Resources Evaluation Of The Hydrocarbon-rich Generation Depression With Sparse Wells

A large petroleum system is controlled by the hydrocarbon-rich generation depression, which can be considered that "no hydrocarbon-rich generation depression, no large oil field". Therefore, the evaluation of the petroleum resources in the hydrocarbon-rich generation depression has always been much accounted by the experts. The forming conditions of the hydrocarbon-rich generation depression include three aspects:the long-term non-compensation environment (rift), the abundant organic matter provenance (rich in the phytoplankton and the nutrition water) and the favorable preservation conditions of the organic matter (oxygen-poor environment). Then the fundamental geological features are high deposition rate, high hydrocarbon generation intensity, anomaly overpressure, and favorable source-reservoir-cap assemblage. Based on the above understanding, the resources of the hydrocarbon-rich generation depression should be estimated by the genetic method. Because the genetic method can reflect efficiently the genetic parameters of the excellent source rocks of the hydrocarbon generation expulsion, migration and accumulation in the hydrocarbon-rich generation depression, which has the clear geological significances.The organic carbon content of the source rock is the characterization parameters of the abundance of organic matter, and is the base of the evaluation of hydrocarbon generation condition and resource potential in the petroliferous basin. Especially in the deep depression zone of the hydrocarbon generation depressions, the quality of the source rock is generally good with the less drilling or no drilling, which leads to the acquisition of the source rock samples difficultly. In such a case, the organic carbon content will be restricted by the number and distribution of the source rock samples, which is difficult to meet the needs of the research. The measured data distribute in the marginal zone of the depression generally, which can not reveal the organic carbon content and the distribution characteristics in the center of the hydrocarbon-rich generation depression effectively, affecting the resource evaluation greatly. Therefore, it is necessary to use the drilling-seismic prediction technology of the source rock TOC. Through the establishment of the TOC3D data volume, the TOC contents and the distribution characteristics are predicted quantitatively in the3D space in the different depths and different time planes, which makes up the deficiencies of the less coring and the discontinuous distribution of the measured sample, characterizes the distribution characteristics of the source rock in the3D space, and lays a good foundation for the evaluation of the source rock and the calculation of the resources.The characteristic of the hydrocarbon generation rate is also a very important parameter of the resource evaluation, which can be got from the thermo compression simulation experiment. The thermo compression simulation experiment with the formation pore can reserve the original pore structure of the original sample, and the temperature and pressure can be manually controlled giving the similar formation fluid pressure and the overlying rock pressure with the geological conditions, which are more consistent with the prevailing geological background. The experimental data is slightly higher than the data from the thermo compression simulation experiment with the conventional autoclave, but it is more accurate and improves the credibility of the resource evaluation.The Huizhou Depression is a proven typical hydrocarbon-rich generation depression located in the Zhu Yi Depression, Pearl River Mouth Basin. The mainly source rocks are the Paleogene Wenchang and Enping Formations. Although there are many wells in the Huizhou Depression, but the target layers of most of the wells are the above sandstone reservoirs. There are only a fraction of wells drilling the Enping Formation and a few wells drilling the top stratum of the Wenchang Formation. Based on the above analysis, the Huizhou Depression is the typical hydrocarbon-rich generation depression with sparse wells for the source sequence.Using the drilling-seismic prediction technology of the source rock organic carbon and the thermo compression simulation experiments, the key parameters of the petroleum resources evaluation for the target stratum of the Paleogene source rocks in the Huizhou Depression are optimized to consistent with the geological background in the research. Then the hydrocarbon generation amount of the source rock is calculated by the genetic method. The amount of the petroleum resources is calculated in the Huizhou Depression based on the migration and accumulation coefficient, which lays a good foundation for the exploration of the petroleum resources. In addition, through the establishment of the geological model of the hydrocarbon source index, combined with conventional petroleum resources, the grading evaluation criterion of the hydrocarbon source index for the fast evaluation of the hydrocarbon-rich generation depression is presented, which is convenient for the fast evaluation of the resource potential of the source rock in the hydrocarbon-rich generation depression. The mainly achievements as follows:(1) The measured geochemical characteristics of the Paleogene source rock in the Huizhou Depression show that the Lower Wenchang Formation is regarded as a medium source rock, the Upper Wenchang and Enping Formations are regarded as good source rocks, and the quality of the source rock of the Upper Wenchang Formation is superior to that of the Enping Formation.The Lower Wenchang Formation is regarded as a medium source rock by the TOC with the low maturity and the organic matter type Ⅱ2. The Upper Wenchang Formation is regarded as a good source rock by the TOC with the low-medium maturity and the organic matter type Ⅱ1～ Ⅱ2. The Enping Formation is regarded as a good source rock by the TOC with the low-medium maturity and the organic matter type Ⅱ2. It is analyzed comprehensively that the quality of the source rocks of the Upper Wenchang Formation is the best, then that of the Enping Formation, and that of the Lower Wenchang Formation in the last. But the measured data of the Lower Wenchang Formation is lack of the certain representativeness. Because there are less drillings in the Lower Wenchang Formation of the Huizhou Depression, and the drilled strata are the top of the Lower Wenchang Formation, which lead to the acquisition of the source rock samples difficultly. So the quality of the Lower Wenchang Formation need to be further confirmed.(2) The predicted logging curves of the TOC of the Paleogene source rock in the Huizhou Depression show that the TOC of the Wenchang Formation is significantly higher than that of the Enping Formation, which illustrates that the quality of the source rock of the Wenchang Formation is superior to that of the Enping Formation.There is a positive correlation between TOC and the resistivity, acoustic, neutron porosity, natural gamma, and there is a negative correlation between TOC and density. Two predicted logging models of the Wenchang and Enping Formations are established based on the the response relationships, and the TOC curves of the the Wenchang and Enping Formations for eight typical drillings are predicted. The predicted TOC curves fit well with the measured TOC data, which illustrates that this method is feasible. Based on the comprehensive analysis of the TOC curves, the TOC of the Wenchang Formation is higher than that of the Enping Formation, which illustrates that the quality of the source rock of the Wenchang Formation is superior to that of the Enping Formation.(3) The drilling-seismic prediction technology of the source rock TOC is adopted with the bridge of the logging information, which gives a response relationship between the TOC and3D seismic attributes. The TOC3D data volume established shows the TOC contents and the distribution characteristics in the3D space in the different depths and different time planes of the hydrocarbon generation depression.In this technology, the TOC curves is predicted quantitatively by five logging parameters firstly; then the fitting relationship between TOC and many seismic attributes such as amplitude envelope, absolute amplitude integral average frequency, instantaneous phase, wave impedance; the TOC3D data volume is established based on the fitting relationship and3D data volume at last. The TOC distribution characteristics can be showed by the depth and time slices of the TOC3D data volume, which makes up the deficiencies of the less coring and the discontinuous distribution of the measured sample, and can give the thickness and the distribution range of the source rock. Compared with the previous technology, this technology has greater advantages. Fistly, many seismic attributes are used; secondly, not only the TOC contents are predicted quantitatively, but also the thickness and the distribution range of the source rock are predicted.(4) Through the analysis of the TOC3D data volume of the Huizhou Depression, the quality of the source rock of the Lower Wenchang Formation is superior to that of the Upper and Enping Formations, and the hydrocarbon generation potential of the Lower Wenchang Formation is the largest. The thickness and maturity of the effective source rock of the Wenchang Formation are bigger than that of the Enping Formation.The TOC3D data volume of the Huizhou Depression shows the distribution characteristics of the TOC in the3D space. The TOC distribution maps of the third-order sequences of the PSQ2, PSQ3, PSQ4, PSQ6, PSQ9, PSQ10sequences show that the TOC contents of the Upper and Lower Wenchang Formations are obviously higher than that of the Enping Formation. The quality of the Lower Wenchang Formation with the TOC contents ranged mainly from2.5%to3.5%is superior to that of the Upper Wenchang Formation with the TOC contents ranged mainly from2%to3%and the Enping Formation with the TOC contents ranged mainly ranged from1.5%to2.5%, which illustrates that the Lower Wenchang Formation has a largest hydrocarbon generation potential. In addition, The thickness and maturity distribution maps of the third-order sequences of the PSQ2, PSQ3, PSQ4, PSQ6, PSQ9, PSQ10sequences show that the thickness and maturity of the Upper and Lower Wenchang Formations are obviously higher than that of the Enping Formation.(5) The characteristics of hydrocarbon generation rate of the Enping, Upper and Lower Wenchang Formations in the Huizhou Depression show that the hydrocarbon generation rate and the hydrocarbon generation potential of the Upper Wenchang Formation are the highest, followed by that of the Enping and Lower Wenchang Formation.The results are similar to the measured geochemical characteristics of source rocks limited to the number of the source rock samples of the Lower Wenchang Formation, which can not reflect the characteristics of the hydrocarbon generation rate of the Lower Wenchang Formation. So the characteristics of the hydrocarbon generation rate of the Upper Wenchang Formation should be used in the calculation of the amount of the hydrocarbon generation on behalf of that of the Upper and Lower Wenchang Formations. In addition, the best selection is the thermo compression simulation experiment with the formation pore. The experiment can reserve the original pore structure of the original sample, and the temperature and pressure can be manually controlled giving the similar formation fluid pressure and the overlying rock pressure with the geological conditions, which are more consistent with the prevailing geological background. The experimental data is slightly higher than the data from the thermo compression simulation experiment with the conventional autoclave, but it is more accurate and improves the credibility of the resource evaluation.(6) Based on the drilling-seismic prediction technology of the source rock TOC and the thermo compression simulation experiment, the genetic method is used in the hydrocarbon-rich generation depression with sparse wells.The amount of petroleum resources mainly depends on the quality of the source rocks, so the genetic method is appropriate to use in the evaluation of the petroleum resources. The drilling-seismic prediction technology of the source rock TOC can make up the deficiencies of the less coring and the discontinuous distribution of the measured sample in the hydrocarbon-rich generation depression with sparse wells. The thermo compression simulation experiment with the formation pore should be preferred, which is more accurate and improves the credibility of the resource evaluation. In addition, the method of the petroleum evaluation based on the genetic method, the drilling-seismic prediction technology of the source rock TOC and the thermo compression simulation experiment in the hydrocarbon-rich generation depression with sparse wells can also be applied in the early exploration of the basin. Because of sparse wells, the early evaluation of petroleum resources can serve for the future exploration by this method.(7) The hydrocarbon-rich generation sags are the base of the formation of the hydrocarbon-rich generation depression. The petroleum resources evaluation of the hydrocarbon-rich generation sags have an important guiding role in the petroleum resources evaluation and exploration potential analysis of the hydrocarbon-rich generation depression.There are generally many hydrocarbon generation centers in the hydrocarbon-rich generation depression developing many hydrocarbon-rich generation sags. The hydrocarbon-rich generation depression is mainly controlled by the hydrocarbon generation, and the amount of the petroleum resources has a close relationship with the petroleum resources of the hydrocarbon-rich generation sags. So the petroleum resources evaluation of the hydrocarbon-rich generation sags have an important role in the petroleum resources evaluation and exploration potential analysis of the hydrocarbon-rich generation depression. For the case of the hydrocarbon-rich generation depression of the Huizhou Depression, the amount of the total petroleum resources with the total petroleum resources are about70.3404×108t, which are formed by the petroleum resources of many hydrocarbon-rich generation sags. The resources of the HZ26Sag are the largest, representing14.179%of the total resources. In addition, Two lower Paleogene tectonic transfer zones of the XJ30-HZ26and HZ26-HZ22close the HZ26Sag have the good advantage of the nearly source for hydrocarbon, which has a guiding role in the petroleum exploration of the Huizhou Depression.(8) The geological model of the hydrocarbon source index is established, and the grading evaluation criterion of the hydrocarbon source index for the hydrocarbon-rich generation depression is established, which can be used to fast evaluate the hydrocarbon-rich generation depression.The comprehensive evaluation of the source rock conditions is the base of the petroleum evaluation of the hydrocarbon-rich generation depression. The conventional evaluation methods from data acquisition to the experimental analysis have large investment, long cycle and weak comprehensiveness. The hydrocarbon source index can fast evaluate the hydrocarbon-rich generation depression through the comprehensive analysis of several source rock conditions. Based on the area, maximum thickness, facies, organic matter type, maximum depth, threshold depth and other basic parameters of the hydrocarbon-rich generation depression, the geological model of the hydrocarbon source index is established, the hydrocarbon source indexes are calculated, and the grading evaluation criterion of the hydrocarbon source index for the hydrocarbon-rich generation depression is confirmed. The quality of the source rock and the petroleum resources can be evaluated roughly by this criterion. Because the involved parameters are easy to access, especially, the criterion suit to use in the low level exploration areas.