Detailed sedimentary facies of a sandstone reservoir in the eastern zone of the Sulige gas field, Ordos Basin

Given sustaining exploration, the eastern zone of the Sulige gas field may soon become a key area of exploitation. In order to explore its genesis, types and distribution of the reservoir sandstones in the eastern zone of this gas field, we focused in our study on the provenance and detailed sedimentary facies of sandstone of the He8 （the eighth part of the Shihezi formation, Permian system） and Shanxil （the first part of the Shanxi formation, Permian system） members, based on core observations, analyses in petrography, granularity and logging. The results show that： 1） the sandstone provenance of Shanxil and He8 in the eastern zone of the Sulige gas field is from the north of the Ordos Basin, characterized by dual directions from the north and northeast. 2） The He8 and Shanxil members were deposited in a fluvial-delta sedimentary system. The He8 was mainly deposited in braided rivers, in- cluding braided channels, channel bars, levee and floodplain sub-environments, whereas the Shanxil Member was deposited in braided rivers and deltas, including braided channels, channel bars, floodplains, tributaries and inter-tributary sub-environments. 3） Sedimentary facies bands migrated in drastic fashion towards the basin from the Shanxil to the He8 Member. Base levels of sedi- mentation generally present a trend of small increases in-amplitude, large decreasing amplitudes and slow and gradual Increases. 4） The continuity of the reservoir sandbodies along the source direction is better than that perpendicular to the direction. Compared with Shanxil, both dimensions and continuity of the sandbodies in He8 are better from which we conclude that it is the most fa- vorable part of the reservoir.

Depositional and Diagenetic Controls on Sandstone Reservoirs with Low Porosity and Low Permeability in the Eastern Sulige Gas Field, China

In order to determine the genesis and the factors that control the low-porosity and low- permeability sandstone reservoirs in the eastern Sulige Gas Field in the Ordos Basin, systematic studies on the sedimentary facies and diagenesis were conducted by means of analysis of cores, thin sections, fluid inclusions, X-ray diffraction, cathode luminescence and scanning electron microscope. It was found that the sand bodies of the major gas reservoirs in the Shan1 section （P1S1） and the He8 section （P2H8） were formed during the Permian as sedimentary facies such as braided-channel bars, braided-river channels and point bars of a meandering river. Four types of diagenetic facies developed subsequently： in order from the best to the poorest properties these are type A （weak compaction, early calcite cement-chlorite film facies）, type B （moderate compaction, quartz overgrowth-feldspar corrosion-kaolinite filling facies）, type C （strong compaction, late calcite cement-quartz corrosion facies） and type D （matrix filling and strong compaction facies）. This diagenesis is undoubtedly the main reason for the poor reservoir properties of sandstone reservoirs, but the sedimentary facies are the underlying factors that greatly affect the diagenesis and thus the reservoir performance. Favorable diagenetic facies developed mainly in relatively small lithofacies such as braided-river channels, channel bars and point bars. The vertical distribution of the physical properties and the diagenetic facies of the reservoirs are related to the stratigraphic succession. Most of the sandstones between mudstones and thin beds of sandstone are unfavorable diagenetic facies. Analyses indicate that siliceous cementation can hardly be stopped by hydrocarbon filling. Authigenic chlorite could hardly protect the primary porosity. It not only occupies pore space, but also blocks pathways through sandstone reservoirs, so that it has significant influence on the permeability. Authigenic chlorite cannot be used as a marker for a specific sedimentary facies because it can be formed in different sedimentary facies, but it indicates high hydrodynamic conditions and presence of favorable reservoirs.

Diagenesis and Their Succession of Gas-bearing and Non-gas-bearing Reservoirs in the Sulige Gas Field of Ordos Basin,China

Comparisons have been made among lithology, diagenesis, and reservoir characteristics of gas-bearing and non-gas-bearing ones in the Sulige gas field of the Ordos Basin based on the laboratory analysis of thin sections, scanning electron microscope, and liquid inclusion of the reservoirs. The reservoirs of the Sulige gas field are now in the middle stage of diagenesis and have undergone compaction, cementation and dissolution. The secondary pore of the reservoir originated from the dissolution of the feldspar and tuff because of the organic acid action from the source rocks during the diagenetic middle stage. Gas-bearing reservoirs are common in soluble pore diagenetic facies of coarse detritus quartzose sandstone, whereas non-gas-bearing ones are common in tense compaction diagenetic facies of mud-bearing medium-fine detritus quartzose sandstone and residual intergranular pore diagenetic facies of mud-bearing medium-coarse detritus quartzose sandstone. The secondary pore is developed in gas-bearing reservoirs of the Sulige gas-field as the medium-coarse grain reservoirs formed in a powerful sedimentary environment and experienced strong dissolution. However, the sediments of fine grain size form the non-gas-bearing reservoirs because of less residual primary pore and secondary pore.

Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China

To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin,the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections.According to detailed analysis of petrologic characteristics,the reservoir rock types include micritic dolomite,grain dolomite and microbialite which have mainly moldic pore,intergranular(dissolved)pore,and(dissolved)residual framework pore as main reservoir space respectively.The study area developed upward-shallowing sequences,with an exposure surface at the top of a single upward-shallowing sequence.The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling,indicating these reservoirs were related to the facies-controlled eogenetic karstification.With the increase of karstification intensity,the reservoirs became worse in physical properties.

Study on the Quality Characteristics of Gas Field Water in Eastern Sichuan

Gas field water is the formation water produced together with natural gas in the process of natural gas exploitation.The gas field water treated is difficult to reach the standard due to its wide sources,numerous types and complex composition.In addition,it can pollute soil,surface water and groundwater.In this study,the quality and pollution characteristics of gas field water in eastern Sichuan were studied by conventional water quality determination and GC-MS.The results show that the main components of gas field water in eastern Sichuan were chloride,COD Cr,SS,anionic surfactant,sulfide and other substances.The gas field water could be divided into two types according to the characteristics of water quality,of which one had high mineralization and high organic compounds,and the other had high sulfur and high organic compounds.There were 17 kinds of organic pollutants in the gas field water,mainly including alkanes,alcohols,esters and a small amount of acids.

A Volumetric Model for Evaluating Tight Sandstone Gas Reserves in the Permian Sulige Gas Field,Ordos Basin,Central China

To accurately measure and evaluate reserves is critical for ensuring successful production of unconventional oil and gas. This work proposes a volumetric model to evaluate the tight sandstone gas reserves of the Permian Sulige gas field in the Ordos Basin. The reserves can be determined by four major parameters of reservoir cutoffs, net pay, gas-bearing area and compression factor Z, which are controlled by reservoir characteristics and sedimentation. Well logging, seismic analysis, core analysis and gas testing, as well as thin section identification and SEM analysis were used to analyze the pore evolution and pore-throat structure. The porosity and permeability cutoffs are determined by distribution function curve,empirical statistics and intersection plot. Net pay and gas-bearing area are determined based on the cutoffs, gas testing and sand body distribution, and the compression factor Z is obtained by gas component. The results demonstrate that the reservoir in the Sulige gas field is characterized by ultralow porosity and permeability, and the cutoffs of porosity and permeability are 5% and 0.15×10^(–3) μm^2, respectively. The net pay and gas-bearing area are mainly affected by the sedimentary facies, sand body types and distribution. The gas component is dominated by methane which accounts for more than 90%, and the compression factor Z of H_8(P_2h_8) and S_1(P_1s_1) are 0.98 and 0.985, respectively. The distributary channels stacked and overlapped, forming a wide and thick sand body with good developed intergranular pores and intercrystalline pores. The upper part of channel sand with good porosity and permeability can be sweet spot for gas exploration. The complete set of calculation systems proposed for tight gas reserve calculation has proved to be effective based on application and feedback. This model provides a new concept and consideration for reserve prediction and calculation in other areas.

Application of wide-field electromagnetic method in shale gas survey in Eastern Kunlun,Qinghai province,China

In order to study the distribution of shale gas reservoir in the Babaoshan Basin of Eastern Kunlun,the wide-field electromagnetic(WFEM)survey was carried out to obtain the spatial distribution characteristics of the underground electrical volume resistivity based on the delineation of the scope of the Babaoshan Basin by regional gravity data.The basic characteristics of the basement,basin framework,and extension,vertical change,burial depth of dark mud shale in this area were identified,and the electrical distribution of the Babaoshan mud shale horizon was revealed,which has been proved to be a good geological effect by drilling.The exploration results show that the WFEM has significant effects on the exploration of shale gas occurrence strata,which meets the needs of investigation and evaluation of multi-layered and large-scale shale gas,and plays a good demonstration role in the follow-up shale gas exploration.

Technical strategies for effective development and gas recovery enhancement of a large tight gas field: A case study of Sulige gas field, Ordos Basin, NW China

Based on the analysis of influencing factors of tight gas recovery and reservoir geological characteristics, the types of remaining tight gas reserves in the Sulige gas field are summarized from the perspective of residual gas genesis to estimate residual gas reserves of different types and provide corresponding technical strategies for enhancing gas recovery. The residual gas reserves in the Sulige gas field can be divided into four types: well pattern uncontrollable, horizontal well missing, imperfect perforation, blocking zone in composite sandbodies. Among them, the uncontrolled remaining gas of well pattern and blocking zone in composite sandbodies are the main body for tapping potential and improving recovery factor, and well pattern infilling adjustment is the main means. Taking into account reservoir geological characteristics, production dynamic response and economic benefit requirements, four methods for infilling vertical well pattern, i.e., quantitative geological model method, dynamic controlled range of gas well method, production interference method and economic and technical index evaluation method, as well as a design method of combined vertical well pattern with horizontal well pattern are established. Under certain economic and technological conditions, the reasonable well pattern density of enrichment zone of gas field is proved to be 4 wells per square kilometers, which can increase the recovery rate of the gas field from 32% to about 50%. Meanwhile, five matching techniques for enhancing gas recovery aimed at interlayer undeveloped residual gas have been formed, including tapping potential of old wells, technological technology optimizing of new wells, rational production system optimizing, drainage and gas producing, and reducing waste production, which could increase the recovery rate for 5% based on well pattern infilling. The research results provide effective support for the long-term stable production of 230×108 m3/a of the Sulige gas field and production growth in the Changqing gas area.

Gas expansion caused by formation uplifting and its effects on tight gas accumulation:A case study of Sulige gas field in Ordos Basin,NW China

Gas expansion caused by significant exhumation in the Sulige gas field in the Ordos Basin since Late Cretaceous and its effects on hydrocarbon accumulation have been investigated systematically based on comprehensive analysis of geochemical,fluid inclusion and production data.The results indicate that gas volume expansion since the Late Cretaceous was the driving force for adjustment and secondary charging of tight sandstone gas reservoirs in the Sulige gas field of the Ordos Basin.The gas retained in the source rocks expanded in volume,resulting in gas re-expulsion,migration and secondary charging into reservoirs,while the gas volume expansion in the tight reservoirs caused the increase of gas saturation,gas-bearing area and gas column height,which worked together to increase the gas content of the reservoir and bring about large-scale gas accumulation events.The Sulige gas field had experienced a two-stage accumulation process,burial before the end of Early Cretaceous and uplifting since the Late Cretaceous.In the burial stage,natural gas was driven by hydrocarbon generation overpressure to migrate and accumulate,while in the uplifting stage,the gas volume expansion drove internal adjustment inside gas reservoirs and secondary charging to form new reservoirs.On the whole,the gas reservoir adjustment and secondary charging during uplifting stage is more significant in the eastern gas field than that in the west,which is favorable for forming gas-rich area.

Practice and understanding of sidetracking horizontal drilling in old wells in Sulige Gas Field, NW China

To seek effective ways of lowering development cost and tapping inter-well remaining reserves, sidetracking horizontal wells from old wells in Su10 and Su53 Block were conducted. The engineering and geological problems such as leakage, collapse and sticking in slim-hole sidetracking, and difficult evaluation of remaining gas were gradually overcome, and a set of drilling and completion technology, well deployment optimization technology and geo-steering technology suitable for sidetracking horizontal wells in tight sandstone gas reservoirs have been worked out. By making full use of the old well, sidetracking horizontal wells can greatly reduce development costs, enhance the producing degree of inter-well remaining reserves, and get production 3-5 times of that of adjacent vertical wells.Its production effect is influenced by encountered sandstone length, the position of the horizontal segment in the reservoir, produced effective reservoir thickness, gas saturation, controlled reserves and fracturing effect, etc. Up to now, in Block Su10 and Su53, 12 sidetracking horizontal wells have been drilled, which have an average drilling cycle of 49 days, average horizontal section length of 689 m,average effective drilling ratio of 61.5%, average well-head pressure of 16.2 MPa, and daily output of 4.7×10~4 m^3 at the initial stage after production. By the end of 2017, the average yield increment was more than 1 000×10~4 m^3 with good effect. With the increase of low yield old wells, wells in the enrichment regions tend to be saturated and the rest gas-bearing areas are lower in grade, therefore, sidetracking horizontal well can be used for optimization of well pattern, well deployment mode and exploitation of remaining oil areas.

Origin and migration model of natural gas in L gas field, eastern slope of Yinggehai Sag, China

Based on the chemical and stable carbon isotopic composition of natural gas and light hydrocarbons, along with regional geological data, the genetic type, origin and migration of natural gases in the L lithologic gas field, the eastern slope of Yinggehai Sag were investigated. The results show that these gases have a considerable variation in chemical composition, with 33.6%–91.5% hydrocarbon, 0.5%-62.2% CO2, and dryness coefficients ranging from 0.94 to 0.99. The alkane gases are characterized by δ13C1 values of -40.71‰--27.40‰,δ13C2 values of –27.27‰– –20.26‰, and the isoparaffin contents accounting for 55%–73% of the total C5–C7 light hydrocarbons. These data indicate that the natural gases belong to the coal-type gas and are mainly derived from the Miocene terrigenous organic-rich source rocks. When the CO2 contents are greater than 10%, the δ13CCO2 values are –9.04‰ to – 0.95‰ and the associated helium has a 3He/4He value of 7.78×10^–8, suggesting that the CO2 here is crustal origin and inorganic and mainly sourced from the thermal decomposition of calcareous mudstone and carbonate in deep strata. The gas migrated in three ways, i.e., migration of gas from the Miocene source rock to the reservoirs nearby;vertical migration of highly mature gas from deeper Meishan and Sanya Formations source rock through concealed faults;and lateral migration along permeable sandbodies. The relatively large pressure difference between the “source” and “reservoir” is the key driving force for the vertical and lateral migration of gas. Short-distance migration and effective “source - reservoir” match control the gas distribution.

Diagenetic mineralogy and its effect on the reservoir properties of the sandstones of the Permian of S120 block(Sulige gas field),Ordos Basin,NW China

The characteristics of diagenetic minerals and their effects on reservoir quality of the tight sandstones of the Permian in Sulige gas field of the Ordos Basin were studied through observations on thin sections,scanning electron microscopy,energy dispersive spectroscopy analysis and electron microprobe analysis.Diagenetic minerals in the Permian sandstones consist of illite,kaolinite,chlorite,siliceous and calcite cements.Large amounts of intercrystalline pores between kaolinite and illite provide channels for acidic fluids flow and thereby were conducive to the formation of clastic solution pores,intergranular solution pores and composite pores.Authigenic chlorite occurs in the form of three morphotypes as grain-coating,pore-lining and pore-filling.Grain-coating and pore-lining chlorite with different crystal shapes occur as coatings on the framework grains.Pore-filling chlorite precipitated as discrete flaky plates in pore spaces.Quartz microcrystals developed but quartz overgrowth did not develop because of the occurrence of porelining chlorite.This,in turn,led to the preservation of primary pores by occupying potential nucleation sites for quartz overgrowth and thereby preventing quartz microcrystals from merging into quartz overgrowth.This process is regarded as the most important for influencing the quality of the lithophysical properties.Calcite cement was mainly precipitated during a late diagenetic stage and has a negative effect on the reservoir quality.This study provides important insights into analyzing the relationship between diagenetic minerals and reservoir quality and the results are directly applicable to the exploration and development of tight sandstone reservoirs all over the world.

Sulige Gas Field super project

Owner:Petro China Changqing Oilfield Company Design unit:Petro China Changqing Oilfield Company Construction unit:Petro China Changqing Oilfield Company,CNPC Chuanqing Drilling Engineering Company Limited,CNPC Greatwall Drilling Company。

鄂尔多斯盆地苏里格气田东区致密储层分布模式

苏里格气田是我国典型的致密气储层发育区,对其储层沉积类型和成岩作用过程历来多有研究且存在争议。利用岩心实验及分析资料,研究了苏里格气田东区致密储层地质特征,从沉积和成岩2个方面阐述了储层物性差异,并指出强压实、铁方解石交代和硅质胶结等作用是导致该区储层致密化的主要因素。在分析沉积微相的基础上,明确了分流河道砂体是苏里格气田东区主要的优质储层,并建立了三角洲平原分流河道单砂体的4种垂向组合模式和3种侧向接触模式。

鄂尔多斯盆地苏里格气田东部优质储层分布规律

分析优质储层的分布规律及其控制因素,对于加快天然气的勘探开发进程具有十分重要的现实意义。通过岩心观察、铸体薄片鉴定、粒度分析、扫描电镜、毛管压力分析以及包裹体测温等分析测试手段,根据成岩作用和成岩相特征,探讨鄂尔多斯盆地苏里格气田东部优质储层的分布规律。研究表明,碎屑颗粒粒度、砂岩类型以及成岩作用的差异是影响储层物性的主要因素。研究区目的层可划分出9种成岩相带,主要以优质储层与非优质储层的组合形式出现。位于致密相之间的石英加大胶结混合孔隙相、自生高岭石胶结晶间孔相以及粘土杂基混合充填溶蚀相,是发育优质储层的良好成岩相带。微裂缝的发育以及溶解作用的发生,是形成优质储层的关键因素。

苏里格气田东区致密砂岩储层物性下限值的确定

长庆油田苏里格气田东区盒8、山1、山2段致密砂岩储层具有典型的低孔、低渗及多层系含气特征。储层下限标准的研究是划分有效储层的基础,也是气层识别和容积法储量计算的前提。采用经验统计法、测井计算-Sg法、孔隙结构参数分布等几种方法对研究区储层物性下限进行研究。结果表明,地面条件下有效储层的孔隙度下限为5.0%,渗透率为0.10×10-3～0.15×10-3μm2,含气饱和度下限为50%～60%;原始地层温度压力条件下,孔隙度下限为4.5%,渗透率仅为0.001 0×10-3～0.001 5×10-3μm2。研究结果得到了试气试采资料的证实,为储量计算提供了依据,对储层的开发生产具有指导作用。

苏里格气田东二区致密砂岩储层裂缝与含气性的关系

鄂尔多斯盆地苏里格气田东区砂岩储层岩性致密,已成为制约天然气规模开发的主要因素。为此,在岩心观察的基础上,采用薄片鉴定、扫描电镜、铸体、压汞等分析方法,对该气田东二区下二叠统山西组山1段和下石盒子组盒8段的储层进行了分析。结果发现:研究区北部砂岩储层中裂缝(裂缝类型主要为构造裂缝和成岩裂缝)较发育,对储层物性有明显的改善作用,而南部则未见到构造裂缝;构造裂缝与溶蚀裂缝均沿构造带边界分布,是致密储层渗透率提高的重要因素;裂缝发育层段,砂岩含气性明显变好。因此,把握裂缝的分布规律对于储层预测、寻找高产气藏、搞好天然气开发设计与施工等具有重要意义。

苏里格气田东区盒8段-山1段-山2段储集层致密化差异性及影响因素研究

苏里格气田东区具有多层系含气特征,其中盒8段、山1段、山2段是该区主力产层,都呈现出低孔-特低孔,特低渗-超低渗这一典型致密储集层特征,由于差异性致密化,三层段的物性参数、孔隙结构等表现出较大的差异。综合应用岩石铸体薄片、扫描电镜、高压压汞、常规物性、X射线衍射、流体包裹体测温、成岩演化序列重建,以及多元回归分析等方法,对三层段的储集层特征、成岩作用、成岩演化序列及储集层致密化影响因素进行了定性、定量评价。结果表明,三层段储集层砂岩主要为富岩屑砂岩,经历的成岩演化序列具有一定相似性,孔隙演化与各段强度差异较大的成岩作用密切相关;储集层致密化时间盒8段为中成岩A期末,山1段为中成岩B期初,山2段为中成岩B期中期;定量分析了影响孔隙度和渗透率对各段物性的影响程度,并以此结果指导优选勘探区带。

PDC钻头在苏里格气田东区的应用

在苏里格气田东区开发中要始终坚持低成本开发的指导思想下,以PDC钻头应用及优化钻井参数技术为核心大幅度降低了钻井成本,成为苏里格气田控制投资的关键技术之一。文章通过对PDC钻头及优化参数与该区块地层情况相结合而总结了优选PDC钻头技术。通过PDC钻头的应用,真正实现了"优质、安全、高效、低耗"的钻井施工目的,必将推动苏里格气田东区的快速发展。

苏里格气田东区二叠系盒8段上部沉积相研究

综合运用岩心、测井等资料,对鄂尔多斯盆地苏里格气田东区二叠系盒8段上部沉积相特征进行了研究。研究结果表明,盒8段上部主要发育洪泛-曲流河沉积体系,主要沉积微相有曲流河道、边滩、河漫滩、河道间等微相类型;曲流河沉积主要呈北北东向,可以识别出3大主河道带;演化至盒8段晚期,盒8段上部物源体系由早期的北北东向充沛物源体系转变为目的层段的物源供给欠充分的物源体系。