Fine geological modeling leads to accurate reservoirs numerical simulations. Fractured biogenic limestone has abundant storage spaces and flow paths to accumulate oil and gas. The complexity and diversity of fractured biogenic limestone also lead to challenges in accurately characterizing its pore volume and remaining oil. This investigation aimed to enhance the understanding of fractured biotite reservoir properties via geological modeling. Numerical simulations were used to characterize the remaining oil during the late stage of field development. Considering the differences in porosity and permeability between fractures and matrix, a facies-controlled stochastic modeling technique was used to establish a dual-porosity and dual-permeability (DPDP) model for numerical simulation. Core information, logging data, and multiple seismic attributes were combined to guide low-level sequence fault interpretation for tectonic refinement. Based on classified seismic inversion, sedimentary phases were reconstructed. A discrete fracture network (DFN) model was obtained based on fracture occurrences and density models. The optimized discrete adjoint (ODA) algorithm was utilized to calibrate model parameters. The findings revealed that dense tectonic fractures develop in thick biogenic limestone areas. Combined with advanced reservoir simulation technology, these findings suggest that areas of thicker biogenic limestone were consistent with areas of higher fracture matrix conductivity multipliers. The remaining oil distribution patterns were investigated, and to deploy new wells was guided. Therefore, it is essential to better understand the tectonic characteristics of fractured biogenic limestone reservoirs and their remaining oil distribution patterns by integrating multiple sources of information and mastering advanced reservoir simulation technology for oilfield development.

Preventing eutrophication requires a deep understanding of nutrient sources and migration processes. The Guanshan River in the Danjiangkou Reservoir was selected as a typical agricultural basin. River water, sediment, and soil samples were collected to determine nitrogen (N) and phosphorus (P) contents and forms. Multivariate statistical analysis, buffer analysis, and extreme gradient boosting regression (XGBoost) were used to investigate the nutrient sources, the sources’ scale effects, and migration. The results showed that the exogenous sources of river nutrients were fertilizer (provided phosphate (PO4-P) and ammonium (NH4-N)), domestic wastewater (provided nitrate (NO3-N) and organic nitrogen), and natural soil and rock leaching (provided PO4-P and organic phosphorus). Fertilization within 300–1200 m and domestic wastewater discharge within 150 m of the river affected the contents of the river nutrients significantly (with R ranging between 0.40 and 0.73, p ≤ 0.01). The sediment was the N source and P sink of the overlying water. With NH4-N/PO4-P compound fertilization, the NO3-N and calcium-bound phosphorus (Ca-P) exhibited co-enrichment. Appropriately increasing NH4-N application could stimulate P biological uptake, thus inhibiting P emigration from agricultural soil under N-limited states. In conclusion, this study effectively recognized river nutrient sources and their scale impacts and also explored more effective fertilization strategies, which are beneficial for the optimized management of agricultural activities.

Acid fracturing is an effective stimulation technology that is widely applied in carbonate reservoirs. An integrated model for acid fracturing without prepad treatment has been established. Compared with the previous models which use prepad for generating hydraulic fractures, this model can simultaneously simulate the fracture propagation and the acid etching of fracture surfaces, as well as the wormhole growth during acid fracturing. The influences of some essential factors have been studied through a series of numerical simulations, and the main conclusions are as follows. First, increasing the injected acid volume can expand the size of the formed hydraulic fractures and extend the propagation distance of the wormhole. Increasing the injected acid volume can also expand the etched width and extend the effective distance of the injected acid. Second, a high injection rate impels more acid to flow into the depth of a fracture before infiltration and reaction, resulting in the augmentation of a hydraulic fracture’s geometric size and the extension of the effective distance. But the maximum etched width decreases as the injection rate rises. A high injection rate can also enable wormholes to grow in the natural fracture area farther away from the hydraulic fracture inlet, but shorten the length of the original wormhole near the hydraulic fracture inlet. Third, an increase in acid viscosity can enlarge the geometric size of the hydraulic fracture and reduce the propagation distance of wormholes. In addition, an increase in the acid viscosity blocks the acid flow from fracture inlet to tip, reducing the effective distance of acid fracturing. Fourth, the natural fracture is the vital inducement of wormhole growth, and wormholes are apt to grow in the natural fracture area. Moreover, the geometric size of the hydraulic fracture and the effective distance of acid fracturing decrease with an increasing number of natural fractures. This research can provide a reference for field applications of acid fracturing without prepad.

The shale oil reservoirs of the Lower Permian Fengcheng Formation in the northern Mahu Sag are promising targets. However, complex geology and strong heterogeneity in the area pose great difficulties in the numerical simulation of in situ stress fields, which have for a long time been poorly understood. This study provides a systematic and accurate 3D in situ stress numerical simulation workflow based on comprehensive data. In this research, optimized ant tracking was applied to construct refined geological models. Acoustic impedance is taken as what we refer to as “hard” data to reflect variations in geomechanical parameters. Logging and mechanical tests were taken as “soft” data to restrict the numerical range of the geomechanical parameters. With the integration of “hard” data and “soft” data, accurate 3D geomechanical models can be attained. The finite element method was ultimately utilized to simulate the 3D in situ stress field of the Fengcheng Formation. Numerical simulation results reveal that the stress state of the Fengcheng Formation is quite complicated. The magnitude of the horizontal principal stress, horizontal stress difference and horizontal stress difference coefficient are correlated with burial depth, faults, and geomechanical parameters to some degree. The parameter Aφ was introduced in this research to better analyze the stress regime, the result of which demonstrates that the main stress regime in the study region is the reverse faulting stress regime. By evaluating the fault stability, it was found that there is basically no possibility of slippage regarding the faults in northern Mahu Sag. The results of this research provide evidence for well deployment optimization, borehole stability, and so on, all of which are of great significance in hydrocarbon exploration and exploitation.

There are complex strike-slip fault systems in the Ordovician carbonate strata in the T-sh area of Tarim Basin. They are the primary storage space and flow channel of hydrocarbon resources. Scientific evaluation of internal space and conductivity of different parts along the main faults is the decisive factor for efficient development of the T-sh reservoir. Based on the 3-D seismic and the coherent body data, the fault structure models in the reservoir of the S1 and S5M fault zones are established. The original regional geomechanical parameters are determined according to the core testing results and logging data. The local stress field near the fault is studied based on the numerical simulation method. The sliding trend coefficient, expansion coefficient, and comprehensive conductivity coefficient of faults in the reservoir of S1 and S5M fault zones are quantitatively evaluated. The results show that the Ordovician strata in S1 and S5 fault zones are in a strike-slip faulting stress regime. The vertical stress (S-V) gradient is 0.0243 MPa/m, the maximum horizontal stress (S-H) gradient is 0.0246 MPa/m, the minimum horizontal stress (S-h) gradient is 0.0177 MPa/m, and the orientation of S-h is 166.27 degrees-197.25 degrees. Based on the simulation results, it is found that there is an apparent correlation between the sliding trend coefficient of the faults and mud loss. The expansion coefficient is positively correlated with venting and production. The research results have practical significance for geological sweet spots prediction and drilling construction in the T-sh oilfield.

Karst cavities and caves are often present along fractures in limestone reservoirs and are of significance for oil and gas exploration. Understanding the formation and evolution of caves in fractured carbonate rocks will enhance oil and gas exploration and development. Herein, a reactive transport model was established considering both the matrix and fractures. Different factors affecting the dissolution along fractures were considered in the simulation of matrix–fracture carbonate rocks, including the magnitude and characteristic length of the matrix porosity heterogeneity, intersecting fractures, and complex fracture network. The results show that a strong heterogeneity of the matrix porosity significantly affects the cave formation along the fracture and the existence of fractures increases the heterogeneity due to the high permeability as well as the dissolution area. The characteristic length of the matrix porosity heterogeneity affects the cave location and shape. The larger permeability of intersecting fractures or the matrix greatly increases the cave size, leading to the formation of large, connected cave areas. A complex fracture network leads to more developed karst dissolution caves. The topology of the fracture network and preferential flow dominate the distribution of caves and alleviate the effect of the matrix heterogeneity.

For ultra deep strike-slip faults controlled reservoirs, the permeability along the faults directly determines the distribution of favorable reservoirs. The present in-situ stress has a significant influence on the permeability of the faults. Therefore, it is meaningful to study the relationship between in-situ stress and permeability along the strike-slip faults for exploration and development of Shunbei oilfield. Based on seismic and coherent data, a three-dimensional model of strike slip fault in reservoir is established. The size and direction of in-situ stress are determined by rock mechanics test. Combined with drilling history and imaging logging data of typical wells, natural fractures are analyzed. Finally, the slip tendency coefficient of Shunbei 5 strike-slip fault zone is simulated by using the finite element method. It is found that there is a positive correlation between the slip tendency coefficient and permeability along the strike-slip fault. The critical slip tendency coefficient of Shunbei 5 fault zone is 0.35 (the fault is in the critical open state). This study provides a reference for the prediction of favorable reservoir distribution and the selection of drilling targets for Shunbei ultra deep strike-slip fault zones.

1 Introduction The Cretaceous/Tertiary （Paleogene） extinction event was a mass extinction event occurring at about 65 million years ago between the Mesozoic Cretaceous and Cenozoic Tertiary,which is the one closest to today among the five major extinction events in the the geological history period （Renne et al.,2013）.At the end of the Cretaceous,the proportion of species declined gradually,reaching a peak as the species disappeared closer to