Improvements to the fuzzy mathematics comprehensive quantitative method for evaluating fault sealing

Fuzzy mathematics is an important means to quantitatively evaluate the properties of fault sealing in petroleum reservoirs.To accurately study fault sealing,the comprehensive quantitative evaluation method of fuzzy mathematics is improved based on a previous study.First,the single-factor membership degree is determined using the dynamic clustering method,then a single-factor evaluation matrix is constructed using a continuous grading function,and finally,the probability distribution of the evaluation grade in a fuzzy evaluation matrix is analyzed.In this study,taking the F1 fault located in the northeastern Chepaizi Bulge as an example,the sealing properties of faults in different strata are quantitatively evaluated using both an improved and an un-improved comprehensive fuzzy mathematics quantitative evaluation method.Based on current oil and gas distribution,it is found that our evaluation results before and after improvement are significantly different.For faults in＂best＂and＂poorest＂intervals,our evaluation results are consistent with oil and gas distribution.However,for the faults in＂good＂or＂poor＂intervals,our evaluation is not completelyconsistent with oil and gas distribution.The improved evaluation results reflect the overall and local sealing properties of target zones and embody the nonuniformity of fault sealing,indicating the improved method is more suitable for evaluating fault sealing under complicated conditions.

Quantitative evaluation of lateral sealing of extensional fault by an integral mathematical-geological model

To evaluate the lateral sealing mechanism of extensional fault based on the pressure difference between fault and reservoir, an integral mathematical-geological model of diagenetic time on diagenetic pressure considering the influence of diagenetic time on the diagenetic pressure and diagenetic degree of fault rock has been established to quantitatively calculate the lateral sealing ability of extensional fault. By calculating the time integral of the vertical stress and horizontal in-situ stress on the fault rock and surrounding rock, the burial depth of the surrounding rock with the same clay content and diagenesis degree as the target fault rock was worked out. In combination with the statistical correlation of clay content, burial depth and displacement pressure of rock in the study area, the displacement pressure of target fault rock was calculated quantitatively. The calculated displacement pressure was compared with that of the target reservoir to quantitatively evaluate lateral sealing state and ability of the extensional fault. The method presented in this work was used to evaluate the sealing of F_(1), F_(2) and F_(3) faults in No.1 structure of Nanpu Sag, and the results were compared with those from fault-reservoir displacement pressure differential methods without considering the diagenetic time and simple considering the diagenetic time. It is found that the results calculated by the integral mathematical-geological model are the closest to the actual underground situation, the errors between the hydrocarbon column height predicted by this method and the actual column height were 0–8 m only, proving that this model is more feasible and credible.

Gulong shale oil enrichment mechanism and orderly distribution of conventional–unconventional oils in the Cretaceous Qingshankou Formation,Songliao Basin,NE China

Through the study of organic matter enrichment,hydrocarbon generation and accumulation process of black shale of the Cretaceous Qingshankou Formation in the Songliao Basin,the enrichment mechanism of Gulong shale oil and the distribution of conventional–unconventional oil are revealed.The Songliao Basin is a huge interior lake basin formed in the Early Cretaceous under the control of the subduction and retreat of the western Pacific plate and the massive horizontal displacement of the Tanlu Fault Zone in Northeast China.During the deposition of the Qingshankou Formation,strong terrestrial hydrological cycle led to the lake level rise of the ancient Songliao Basin and the input of a large amount of nutrients,resulting in planktonic bacteria and algae flourish.Intermittent seawater intrusion events promoted the formation of salinization stratification and anoxic environment in the lake,which were beneficial to the enrichment of organic matters.Biomarkers analysis confirms that the biogenic organic matter of planktonic bacteria and algae modified by microorganisms plays an important role in the formation of high-quality source rocks with high oil generation capability.There are four favorable conditions for the enrichment of light shale oil in the Qingshankou Formation of the Gulong Sag,Songliao Basin:the moderate organic matter abundance and high oil potential provide sufficient material basis for oil enrichment;high degree of thermal evolution makes shale oil have high GOR and good mobility;low hydrocarbon expulsion efficiency leads to a high content of retained hydrocarbons in the source rock;and the confinement effect of intra-layer cement in the high maturity stage induces the efficient accumulation of light shale oil.The restoration of hydrocarbon accumulation process suggests that liquid hydrocarbons generated in the early(low–medium maturity)stage of the Qingshankou Formation source rocks accumulated in placanticline and slope after long-distance secondary migration,forming high-quality conventional and tight oil reservoirs.Light oil generated in the late(medium–high maturity)stage accumulated in situ,forming about 15 billion tons of Gulong shale oil resources,which finally enabled the orderly distribution of conventional–unconventional oils that are contiguous horizontally and superposed vertically within the basin,showing a complete pattern of“whole petroleum system”with conventional oil,tight oil and shale oil in sequence.

Features of the fault system and its relationship with migration and accumulation of hydrocarbon in Liaodong Bay

The fault system of Liaodong Bay developed extensively under the control of the Tanlu Fault. The fault system can be grouped into strike-slip faults of grade Ⅰ, trunk faults of grade Ⅱand branch faults （induced faults） of grade Ⅲ respectively based on its developmental scale. The faults of grade Ⅰ and Ⅱwere deep, early and large while the faults of grade Ⅲwere shallow, late and small. The formation, evolution and distribution features played a significant role in controlling the migration of oil and gas in both horizontal and vertical directions. The fluid transfer in the fault system occurred in the process of faulting. The strike-slip and trunk faults moved actively forming predominant pathways for oil and gas migration. The branch faults, with weak activity, generally controlled the development of traps and were beneficial for the accumulation and preservation of oil and gas. The faults of grade Ⅰ and Ⅱ formed the major migration pathways for oil and gas, but their fault activity rates appeared to vary along their strikes. The zones with a relatively low fault activity rate might be favorable for oil and gas accumulation. When the activities of strike-slip, trunk, and branch faults came to a halt, the fault seal behavior had a vitally important effect on the accumulation of oil and gas. The controlling role of the fault over fluid distribution was further analyzed by calculating the fault activity quantitatively.

Methods and Case Studies of Ultra-deep Fault Seal Evaluation

Fault seals are significant for petroleum exploration and production.This study summarizes the fault sealing impacting factors,including lithological juxtaposition,mud smearing,fault rocks and the fault plane stress states,as well as evaluation methods like Allan maps and Shale Gouge Ratio(SGR).The seal evaluation for a wrench fault focuses on its particular structural features.The evaluation methods were applied to the Jinma-Yazihe structure and the Shunbei oilfield.The source rock is the Xujiahe Formation of the Upper Triassic,the reservoirs and caprocks being of the Shaximiao Formation of the Lower Jurassic.The fault sealing evaluations in major faults proved the reservoir formation processes in the wells Jinfo 1(JF1)and Chuanya 609(CY-609),based on the editions of the Allan map showing lithological juxtaposition,the calculation of SGR showing mud smear and analyses of fault stress states.The analyses of stress states were also applied to Shunbei 5 strike-slip fault in the Shunbei area in Tarim Basin.The various sections along the fault were of different mechanical properties,such as compression and extension.Petroleum exploration has demonstrated that the extensional sections are more favorable for oil accumulation than the compressional sections.These evolutionary methods and other understandings will help in analyses of deep fault sealing.

Characteristics of fault zones and their control on remaining oil distribution at the fault edge: a case study from the northern Xingshugang Anticline in the Daqing Oilfield, China

Most major oil zones in the Daqing Oilfield have reached a later,high water cut stage,but oil recovery is still only approximately 35%,and 50%of reserves remain to be recovered.The remaining oil is primarily distributed at the edge of faults,in poor sand bodies,and in insufficiently injected and produced areas.Therefore,the edge of faults is a major target for remaining oil enrichment and potential tapping.Based on the dynamic change of production from development wells determined by the injection-recovery relationship at the edge of faults,we analyzed the control of structural features of faults on remaining oil enrichment at the edge.Our results show that the macroscopic structural features and their geometric relationship with sand bodies controlled remaining oil enrichment zones like the edges of NNE-striking faults,the footwalls of antithetic faults,the hard linkage segments（two faults had linked together with each other to form a bigger through-going fault）,the tips of faults,and the oblique anticlines of soft linkages.Fault edges formed two types of forward microamplitude structures：（1） the tilted uplift of footwalls controlled by inverse fault sections and（2） the hanging-wall horizontal anticlines controlled by synthetic fault points.The remaining oil distribution was controlled by microamplitude structures.Consequently,such zones as the tilted uplift of the footwall of the NNW-striking antithetic faults with a fault throw larger than 40 m,the hard linkage segments,the tips of faults,and the oblique anticlines of soft linkage were favorable for tapping the remaining oil potential.Multi-target directional drilling was used for remaining oil development at fault edges.Reasonable fault spacing was determined on the basis of fault combinations and width of the shattered zone.Well core and log data revealed that the width of the shattered zone on the side of the fault core was less than 15 m in general;therefore,the distance from a fault to the development target should be larger than 15 m.Vertically segmented growth faults should take the separation of the lateral overlap of faults into account.Therefore,the safe distance of remaining oil well deployment at the fault edge should be larger than the sum of the width of shattered zone in faults and the separation of growth faults by vertical segmentation.

Study on characteristics of well-test type curves for composite reservoir with sealing faults

The pressure response for the composite reservoirs with a sealing fault locating in inner and outer region is different,which neglected by previous researchers,would cause significant errors during well-test interpretations.Based on seepage theory,a well-test model of two-region radial composite reservoir with infinite outer boundary has been built in this study considering wellbore storage and skin effects.The solutions for this model and characteristics of the type curves have been analyzed by applying the method of mirror image,Laplace transformation and superposition principle,including a straight fault,a perpendicular fault and parallel faults cases.The study shows that the dimensionless pressure derivative curves would be obviously different in two cases:the well to fault distance is larger,and smaller than the half length of the inner-region radius.Therefore,type curves are presented with reasonable parameters to analyze the distance effect on the dynamic pressure response.The results in this study are of great significance。

Controlling of cements and physical property of sandstone by fault as observed in well Xia503 of Huimin sag,Linnan sub-depression

Cement content of carbonate in tight sandstone near section is much the fault of well Xia503, in the Huimin sag in Linnan sub-depression higher than that of the normal sandstones far away from In order to understand the origin and its impact on fault sealing, analyses of the whole-rock minerals, casting thin sections, cathodoluminescence, isotope and physical properties are conducted on cores from well Xia503. It is found that c~ L3C varies from 0.1%o to 0.6%o with the average value of 0.42%o, c~ LSO varies from -13.5%o to -12.3%c with the average of-13.1%~., and C-O isotope plotting points are distributed in the low to moderate temperature area of the hydrothermal dolomite. According to the occupied relationship, cathodoluminescence, and C-O isotope feature, the carbonate cementation could be divided into four stages： calcites, dolomite, ankerite, and ferrocalcite. It is discovered that the carbonate cementation is negatively related to reservoir physical property, with the porosity of 4.8%, permeability of 0.37 roD, and displacement pressure of 1.97 MPa in the tight sandstone, which have increased by almost one order of magnitude compared to the porosity of 14.3%, permeability of 3.73 mD, and displacement pressure of 0.27 MPa in the normal sandstone, which is far away from the fault. Regardless of the lithology of the counterpart wall of the fault, only the displacement pressure difference caused by carbonate cementation between the tight sandstone and the normal sandstone could seal 41 m high oil column.