Analysis of lithofacies and evaluation of effective reservoirs of member 2 of Xujiahe Formation in the Xinchang area in Western Sichuan

In the Xinchang area in western Sichuan Basin,a total of 121.12 billion cubic meters of proved reserves of natural gas are discovered in Member 2 of Xujiahe Formation,but only 7.739 billion cubic meters of proved reserves of natural gas are produced.The main reason is that sandstones in Member 2 of Xujiahe Formation in the Xinchang area are generally tight,with strong heterogeneity,only intervals with relatively good physical properties have a certain production capacity;therefore,it is very necessary and important to evaluate effective reservoirs in Member 2 of Xujiahe Formation.Through analysis of cores,with consideration of color,sedimentary texture and grain size,lithofacies of Member 2 of Xujiahe Formation in the Xinchang area can be divided into 12 types.According to data of physical properties and thin section analysis,it can be concluded that the favorable lithofacies in the upper submember of Member 2 of Xujiahe Formation are the massive medium-coarse sandstone,the medium-coarse sandstone with cross bedding,and the medium-coarse sandstone with carbonaceous fragments.On the basis of electrical characteristics,well-logging interpretation standards for favorable lithofacies are established,in addition,favorable lithofacies and effective reservoir in the Xinchang area are determined,and distribution of favorable lithofacies(effective reservoir)is identified,which lay a solid foundation for next deployment of appraisal well and development of effective reserves.

Analysis of petrophysical cutoffs of reservoir intervals with production capacity and with accumulation capacity in clastic reservoirs

Methodologies have been developed for calculating cutoffs of reservoir intervals with production capacity （RIPC） and reservoir intervals with accumulation capacity （RIAC） according to the types of pore throat structures and dynamic force by using data from petrophysical analysis, production tests and mercury injection. The data are from clastic reservoirs in the third member （Es3） and the fourth member （Es4） of the Shahejie Formation in the Shengtuo area on the North Slope of the Dongying Sag, Jiyang Depression, China. The method of calculating cutoffs of RIPC is summarized as follows： 1） determination of permeability cutoffs of RIPC; 2） classification of types of pore-throat structures according to mercury injection data and then relating porosity to permeability and determining the relationship between porosity and permeability according to each type of pore-throat structure; and 3） calculating porosity cutoffs of RIPC using established correlation between porosity and permeability according to the type of pore throat structure. The method of calculating cutoffs of RIAC includes： 1） establishing a functional relationship between oil-water interracial tension and formation temperature; 2） calculating limiting values of maximum connected pore-throat radii according to formation temperature and dynamic forces of each reservoir interval; 3） correlating permeability with maximum connected pore-throat radius and then obtaining permeability cutoffs of RIAC; and 4） calculating porosity cutoffs on the basis of permeability cutoffs according to specific correlations, suitable for the type of porethroat structure. The results of this study show that porosity and permeability cutoffs of clastic reservoirs decrease with depth. For a fixed permeability cutoff, the porosity cutoff of R1PC varies because the type of pore throat is different. At a fixed temperature, porosity and permeability cutoffs of RIAC decrease as dynamic force increases. For a fixed permeability cutoff of effective hydrocarbon accumulation, the porosity cutoff also varies with different types of pore throat.

Integrated petrophysical log characterization for tight carbonate reservoir effectiveness: A case study from the Longgang area, Sichuan Basin, China

Ultra-low porosity and permeability, inhomogeneous fracture distribution, and complex storage space together make the effectiveness evaluation of tight carbonate reservoirs difficult. Aiming at the carbonate reservoirs of the Da＇anzhai Formation in the Longgang area of the Sichuan Basin, based on petrophysical experiments and logging response characteristics, we investigated the storage properties of matrix pores and the characteristics of fracture development to establish a method for the characterization of effectiveness of tight reservoirs. Mercury injection and nuclear magnetic resonance （NMR） experiments show that the conventional relationship between porosity and permeability cannot fully reflect the fluid flow behavior in tight matrix pores. Under reservoir conditions, the tight reservoirs still possess certain storage space and permeability, which are controlled by the characteristic structures of the matrix porosity. The degree of fracture development is crucial to the productivity and quality of tight reservoirs. By combining the fracture development similarity of the same type of reservoirs and the fracture development heterogeneity in the same block, a three-level classification method of fracture development was established on the basis of fracture porosity distribution and its cumulative features. According to the actual production data, based on the effectiveness analysis of the matrix pores and fast inversion of fracture parameters from dual laterolog data, we divided the effective reservoirs into three classes： Class I with developed fractures and pores, and high-intermediate productivity; Class II with moderately developed fractures and pores or of fractured type, and intermediate-low productivity; Class III with poorly developed fractures and matrix pores, and extremely low productivity. Accordingly log classification standards were set up. Production data shows that the classification of effective reservoirs is highly consistent with the reservoir productivity level, providing a new approach for the effectiveness evaluation of tight reservoirs.

Effects of reservoirs on seasonal discharge of Irtysh River measured by Lepage test

The Irtysh River is an intemational river partially joining the territories of China, Kazakhstan, and Russia. Cascade reservoirs have been constructed in the upper reaches of the river and their effects on the seasonal discharge of the middle and lower reaches were analyzed considering the mean and dispersion of the seasonal discharge. The Lepage test, which is a nonparametric, two-sample test for detecting location and dispersion, was used to measure the significance of difference between the pre-dam and post-dam seasonal discharge. The results show that the reservoirs＇ effects on the seasonal discharge varied with the season. In the middle reaches of the river, the summer and autumn discharge decreased significantly and their inter-annual variabilities also decreased significantly. The summer and autumn precipitation over the Irtysh River Basin changed little before and after the operation of the reservoir, which indicates that the discharge changes mainly due to water storage of the reservoirs. The reservoirs store water in summer and autumn and store more water in a wet year, which leads to the reduction of the mean and dispersion of the summer and autumn discharge. The winter discharge increased significantly because the reservoirs released water for power generation. The spring discharge changed slightly. In the lower reaches, only the winter discharge increased significantly, and the other seasonal discharge changed slightly. The reservoirs＇ effects on the seasonal discharge are more significant in the middle reaches than in the lower reaches.

How to estimate isotope fractionations of a Rayleigh-like but diffusion-limited disequilibrium process?

The Rayleigh distillation isotope fractionation(RDIF) model is one of the most popular methods used in isotope geochemistry. Numerous isotope signals observed in geologic processes have been interpreted with this model. The RDIF model provides a simple mathematic solution for the reservoir-limited equilibrium isotope fractionation effect. Due to the reservoir effect, tremendously large isotope fractionations will always be produced if the reservoir is close to being depleted. However, in real situations, many prerequisites assumed in the RDIF model are often difficult to meet. For instance, it requires the relocated materials, which are removed step by step from one reservoir to another with different isotope compositions(i.e., with isotope fractionation), to be isotopically equilibrated with materials in the first reservoir simultaneously. This ‘‘quick equilibrium requirement’’ is indeed hard to meet if the first reservoir is sufficiently large or the removal step is fast. The whole first reservoir will often fail to re-attain equilibrium in time before the next removal starts.This problem led the RDIF model to fail to interpret isotope signals of many real situations. Here a diffusion-coupled and Rayleigh-like(i.e., reservoir-effect included) separation process is chosen to investigate this problem. We find that the final isotope fractionations are controlled by both the diffusion process and the reservoir effects via the disequilibrium separation process. Due to its complexity, we choose to use a numerical simulation method to solve this problem by developing specific computing codes for the working model.According to our simulation results, the classical RDIF model only governs isotope fractionations correctly at the final stages of separation when the reservoir scale(or thickness of the system) is reduced to the order of magnitude of the quotient of the diffusivity and the separation rate. The RDIF model fails in other situations and the isotope fractionations will be diffusion-limited when the reservoir is relatively large, or the separation rate is fast. We find that the effect of internal isotope distribution inhomogeneity caused by diffusion on the Rayleigh-like separation process is significant and cannot be ignored. This method can be applied to study numerous geologic and planetary processes involving diffusion-limited disequilibrium separation processes including partial melting,evaporation, mineral precipitation, core segregation, etc.Importantly, we find that far more information can be extracted through analyzing isotopic signals of such ‘‘disequilibrium’’processes than those of fully equilibrated ones, e.g., reservoir size and the separation rate. Such information may provide a key to correctly interpreting many isotope signals observed from geochemical and cosmochemical processes.

Late-Stage Reservoir Formation Effect and Its Dynamic Mechanisms in Complex Superimposed Basins

Complex superimposed basins exhibit multi-stage tectonic events and multi-stage reservoir formation; hydrocarbon reservoirs formed in the early stage have generally late-stage genesis characteristics after undergoing adjustment, reconstruction and destruction of later-stage multiple tectonic events. In this paper, this phenomenon is called the late-stage reservoir formation effect. The late-stage reservoir formation effect is a basic feature of oil and gas-forming reservoirs in complex superimposed basins, revealing not only multi-stage character, relevance and complexity of oil and gas- forming reservoirs in superimposed basins but also the importance of late-stage reservoir formation. Late-stage reservoir formation is not a basic feature of oil and gas forming reservoir in superimposed basins. Multi-stage reservoir formation only characterizes one aspect of oil and gas-forming reservoir in superimposed basins and does not represent fully the complexity of oil and gas-forming reservoir in superimposed basins. We suggest using ＂late-stage reservoir formation effect＂ to replace the ＂late-stage reservoir formation＂ concept to guide the exploration of complex reservoirs in superimposed basins. Under current geologic conditions, the late-stage reservoir formation effect is represented mainly by four basic forms： phase transformation, scale reconstruction, component variation and trap adjustment. The late-stage reservoir formation effect is produced by two kinds of geologic processes： first, the oil and gas retention function of various geologic thresholds （hydrocarbon expulsion threshold, hydrocarbon migration threshold, and hydrocarbon accumulating threshold） causes the actual time of oil and gas reservoir formation to be later than the time of generation of large amounts of hydrocarbon in a conventional sense, producing the late-stage reservoir formation effect; second, multiple types of tectonic events （continuously strong reconstruction, early-stage strong reconstruction, middle-stage strong reconstruction, late-stage strong reconstruction and long-term stable sedimentation） after oil and gas reservoir formation lead to adjustment, reconstruction and destruction of reservoirs formed earlier, and form new secondary hydrocarbon reservoirs due to the late-stage reservoir formation effect.

Influences of environmental factors on the modern 14C reservoir effects in Qinghai-Tibet Plateau lakes

The establishment of reliable age in the lake sediment profile mainly depends on the AMS 14C dating technique.However,the presence of the 14C lake reservoir effects(LREs)restricted for using radiocarbon dating in lake sediment,especially in dry and cold areas with a scarce plant cover in the Qinghai-Tibet Plateau.Hence,the discussion of influence factors of LREs is crucial.This paper selected 15 lakes(17 sediment and 3 plant samples)in the Qinghai-Tibet Plateau to examine the distribution characteristics of the modern LREs and their main influencing factors.In our study area,14 lakes were all affected by the LREs.The minimum 14C year is 5900 a BP towards the deep water area,whereas the maximum 14C year is up to 7185 a BP in the margins of Lake Heihai.The maximum 14C year is up to 7750 a BP,and the minimum 14C year is present-day carbon in the 15 lakes.One further study indicated that the LRE differences in individual lake are mostly owing to the contribution of exogenous carbonate.The results displayed that the LREs tended to increase with the increase of the salinity,moreover,the LREs of saltwater lakes or salt lakes were significantly larger than freshwater lakes due to the possible supply of old total dissolved inorganic carbon with a long residence time in the lakes.Moreover,the contribution of calcite played a significant role on the LREs.Additionally,the LREs differences are affected by the source of organic matter.The lake with groundwater supply shows large LRE due to likely being influenced by crustal and ancient CO_(2) uprising.

Application of Auto-regressive Linear Model in Understanding the Effect of Climate on Malaria Vectors Dynamics in the Three Gorges Reservoir

It is important to understand the dynamics of malaria vectors in implementing malaria control strategies. Six villages were selected from different sections in the Three Gorges Reservoir fc,r exploring the relationship between the climatic ｜：actors and its malaria vector density from 1997 to 2007 using the auto-regressive linear model regressi^n method. The result indicated that both temperature and precipitation were better modeled as quadratic rather than linearly related to the density of Anopheles sinensis.

A quantitative study of the scale and distribution of tight gas reservoirs in the Sulige gas field, Ordos Basin, northwest China

Gas and water distribution is discontinuous in tight gas reservoirs,and a quantitative understanding of the factors controlling the scale and distribution of effective reservoirs is important for natural gas exploration.We used geological and geophysical explanation results,dynamic and static well test data,interference well test and static pressure test to calculate the distribution and characteristics of tight gas reservoirs in the H_(8) Member of the Shihezi Formation,Sulige gas field,Ordos Basin,northwest China.Our evaluation system examines the scale,physical properties,gas-bearing properties,and other reservoir features,and results in classification of effective reservoirs into types Ⅰ,Ⅱ,and Ⅲ that differ greatly in size,porosity,permeability,and saturation.The average thickness,length,and width of type Ⅰ effective reservoirs are 2.89,808,and 598 m,respectively,and the porosity is>10.0%,permeability is>1010^(–3)µm^(2),and average gas saturation is>60%.Compared with conventional gas reservoirs,tight gas effective reservoirs are small-scale and have low gas saturation.Our results show that the scale of the sedimentary system controls the size of the dominant microfacies in which tight gas effective reservoirs form.The presence of different types of interbeds hinders the connectivity of effective sand body reservoirs.The gas source conditions and pore characteristics of the reservoirs control sand body gas filling and reservoir formation.The physical properties and structural nature of the reservoirs control gas–water separation and the gas contents of effective reservoirs.The results are beneficial for the understanding of gas reservoir distribution in the whole Ordos Basin and other similar basins worldwide.

Hybrid Sedimentation in Late Permian-Early Triassic in Western Sichuan Basin,China

Mixed carbonate-siliciclastic sedimentation is common in the Upper Permian-Lower Tri-assic in the western Sichuan（四川） basin.The extensional movement was strong in the Late Permian,resulting in differential uplifting of fault blocks.During this period,there was an extensive retrogres-sion.The Kangdian（康滇） ancient land progressive uplifted and enlarged,forming the major prove-nance of terrigenous clastics.In Changxingian age,mixed terrigenous-carbonate sedimentation was dis-tributed in the area between Dafeishui（大飞水）-well Dashen（大深）-1-well Shoubao（寿保）-1 and Beichuan（北川）-well Guanji（关基）.The terrigenous clastics sourced from Kangdian ancient land input into the relatively shallow water,resulting in an extensive limited platform-mixing tidal flat where moderate-thick laminated grey limestone,thin muddy limestone and purplish red sandy shale were de-posited.The tectonic framework in Feixianguanian（飞仙关） stage was inherited from the Chang-xingian stage.The hybrid sedimentation was limited to the south of Dujiangyan（都江堰）-Xindu（新都）.In Jialingjiangian（嘉陵江） stage,the palaeogeographic features were also unchanged and two complete transgression-retrogression cycles also occurred.The mixing tidal flat was distributed to the west of Baoxing（保兴）-Ya＇an（雅安）-Hongya（洪雅）-Leshan（乐山）,where purplish red sandstone with thin limestone and shale interbeds and grey-dark grey silty mudstone with limestone were deposited.The carbonates in the mixed facies tract have few primary pores preserved and poorly-developed secondary pores due to the relatively high content of terrigenous clastics,strong compaction and weak dissolution.Therefore,they are unfavorable for the development of effective reservoirs.