In Jurassic times, the tectonic context of East Asia changed from a Paleo-Tethys-dominated continent-continent collision and convergence system to an active continental margin subduction system controlled by the Paleo-Pacific tectonic domain; however, the precise timing of this change is uncertain. In this paper, we undertake studies of detrital zircon U-Pb dating and cathodoluminescence (CL) image analysis of sedimentary successions in the Central Sichuan Basin of southwest China to analyze sediment provenance evolution and improve understanding of tectonic evolution. The detrital zircon age spectra document a progressive reorganization of the sediment source system, which becomes particularly evident during deposition of the lower part of the Middle Jurassic Shaximiao Formation (J2s1). The basal sample of J2s1 displays six zircon age groups: including 173-179 Ma (19.4%), 226-260 Ma (19.4%), 350-428 Ma (8.4%), 759-790 Ma (11.1%), 1861-1891 Ma (29.2%), and 2381-2550 Ma (12.5%), with a mixed provenance from the surrounding mountains. The middle and upper samples both display a dominant age peak at 176 Ma (48.9% and 46.2%), which mainly originated from the young magmatic rocks in the Songpan-Ganzi-Longmenshan Orogenic Belt of the southwest sector of the Sichuan Basin. This provenance change can be attributed to the rapid uplift and thrust event of the Songpan-Ganzi-Longmenshan Orogeny, which was linked to far-field effects of the transition from the Paleo-Tethyan tectonic domain to the Paleo-Pacific subduction system in East China and allows these events to be dated to ca. 165 Ma.

The Middle Jurassic Shaximiao Formation in the northwestern Sichuan Basin is widely developed with high-quality tight sandstone reservoirs. However, its provenance system has long been controversial, which has hindered the study of the formation mechanism of high-quality reservoirs. In order to clarify the provenance attribution of the tight sandstone in the Shaximiao Formation and reveal the influence mechanism of provenance evolution on the formation and evolution of tight sandstone reservoirs. This investigation focuses on the Shaximiao Formation in Zitong area of northwestern Sichuan Basin, conducting petrological and elemental geochemical analysis, which systematically analyzing the sedimentary provenance attribution and tectonic environment, and exploring the significance of provenance evolution for the formation of high-quality tight sandstone reservoirs. Analytical results demonstrate that the reservoirs are dominated by lithic arkose and feldspathic litharenite, characterized by moderate to well-sorted grains with metamorphic lithic fragments, ultra-low porosity (average 7.89

Due to the present tectonic and stratigraphic distribution characteristics in the northwest margin of the Sichuan Basin are complex, which restricts the understanding of sequence lithofacies paleogeography of the Middle Permian Maokou Formation. To investigate the process of tectonic–sedimentary evolution, basin-margin outcrop and intra-basin well and seismic data were used. The results show two structural sequences, namely, SSQ1 and SSQ2, and five third-order sequences, namely, SQ1–SQ5, in the Middle Permian Maokou Formation. SSQ1, with stable formation thickness, shows a slow transgression–fast regression sequence structure. SSQ2 shows a fast transgression–slow regression sequence structure in the area with large thickness and a sustained transgression–fast regression sequence structure in the area with small thickness owing to thickness differentiation caused by tectonic subsidence. Sedimentological analysis shows that sequences SQ1–SQ2 mainly consist of carbonate-ramp sediments, the sedimentary facies of which change from intermediate ramp to outer ramp as the area depressed topographically from south to north. Sequences SQ3–SQ5 mainly consist of rimmed carbonate platform sediments; the platform margin turned up along Shuangyushi–Jian’ge–Yuanba, and the region to the north subsided rapidly to form slope-basin facies. Lithofacies paleogeography in the Middle Permian epoch shows that the northwest margin of the Sichuan Basin was dominated by extensional stresses in northeast and northwest directions. Tectonic activities have a dominant control on the distribution of two structural sequences and the evolution of carbonate platform types in the Maokou Formation, and the sea-level change has a dominant control on third-order sequence boundaries and sedimentary facies migration. Thus, the Middle Permian tectono-sedimentary evolution model of the northwest margin of the Sichuan Basin is established. Research findings may offer new ideas and theoretical support to promising facies exploration in the Maokou Formation in the northwest margin of the Sichuan Basin.

为进一步丰富和深化古老深层白云岩储层的成因理论,以四川盆地中部磨溪8井区灯影组四段(灯四段)为例,基于岩心、岩石薄片、测井、储集物性等资料,分析灯四段岩溶特征及演化规律,并探讨了其对储层发育和保存的意义.结果表明,灯四段发育孔洞型储层,储集岩性以微生物白云岩类、岩溶角砾岩和颗粒白云岩为主,岩溶作用具有组构选择性溶蚀、花斑状溶蚀和半离解带等典型早期溶蚀特征.灯四段储层主要集中在向上变浅序列的中—上部,纵向上呈多套薄层状储层频繁叠置、韵律性分布.灯四段优质储层的发育与相对海平面频繁升降驱动的早成岩期岩溶作用密切相关,储层品质与暴露时间和溶蚀强度有关.灯四段储层主要经历了 3期岩溶演化阶段:选择性溶蚀阶段、岩溶分带阶段和整体角砾化阶段.适宜的暴露改造有利于原生残余孔隙的溶蚀扩大,对储层改造具有重要的建设性作用;暴露时间过长或溶蚀强度过大导致早期相对孤立的孔洞彼此连接,但大多垮塌或被充填破坏,表现为整体角砾化.整体上,灯四段有效储层具有"小孔(洞)保存,大孔(洞)充填"的特征,随着岩溶阶段的演化,储集性能先优化后变差.

The accumulation and enrichment of hydrocarbon reflect different degrees of hydrocarbon charging, which is important for hydrocarbon exploration. In this study, we discuss the controlling factors of tight sandstone gas accumulation and enrichment of Xujiahe Formation in Jinhua-Zhongtaishan area, the slope zone of the Western Sichuan Foreland Basin, Southwest China, based on the analysis of the characteristics of the area's source rock, reservoir, and fault, combined with the tectonic background. This study shows that the accumulation and enrichment of tight sandstone gas in the slope zone of foreland basins are controlled by different factors. The natural gas derived from Xu1 Member is easily accumulated in Xu2 Member sandstone reservoir by vertical migration. The high quality source rock generated a large number of natural gas, which preferentially charge into the high quality sandstone reservoir and finally accumulate at the structural high position. The accumulation of tight gas is controlled by high quality source rock (source rock coefficient >10), high-quality reservoir (reserve coefficient >110), and favorable structure (high position of the local structure). When tight sandstone gas meets all prerequisites above for hydrocarbon accumulation, high content of total organic carbon (TOC) benefits enrichment of tight sandstone gas. In addition, the type I faults could be in favor of the natural gas from Xu1 Member to migrate to the upper part of Xu2 Member high quality sandstone reservoir, especially when the natural gas is generated by the middle and lower part of Xu1 Member high quality source rock, which could significantly improve the gas accumulation efficiency and promotes the gas enrichment and high production. In that case, the enrichment of tight gas is controlled by organic matter abundance of source rock (TOC >1.1%) and favorable source-reservoir communication conditions (the development of Type I faults). As a result, the areas with high-quality source rock and sandstone reservoir, favorable structure and migration pathway will be potential targets of tight sandstone gas in the slope zone of foreland basin.

川西前陆盆地上三叠统须家河组是四川盆地致密砂岩天然气(以下简称致密气)勘探的重要领域,其超压体系的形成与演化对其天然气聚集具有重要影响与控制作用.为了深化川西前陆盆地中生界致密气聚集规律认识与有利勘探区带预测,利用该区气藏压力、钻井液密度资料,以及前人开展的包裹体均一温度研究成果与含烃盐水包裹体PVT模拟气藏古压力恢复成果,结合须家河组不同含气区的埋藏史与构造运动,研究了该区地层流体压力特征、形成机制与演化及其与天然气大规模聚集关系.研究结果表明:①川西前陆盆地须家河组于晚三叠世—侏罗纪形成了欠压实作用超压体系,并在烃源岩成熟度为0.75％时消失或维持弱超压;②晚侏罗世—白垩纪的生烃作用形成了须家河组的古超压体系,其中古超压体系在坳陷带形成于中晚侏罗世,在隆起带及其邻区形成于白垩纪;③燕山期—喜马拉雅期的差异构造运动对须家河组超压体系演化影响十分明显,坳陷带—斜坡带在古近纪超压体系得以维持并略有增强,而隆起带与断裂活动强烈区的超压系统遭受不同程度的破坏;④现今超压体系是生烃与构造挤压联合形成的古超压体系经历强烈构造改造后的残余压力体系;⑤超压体系的持续发育促进了川中地区大面积低丰度天然气区的形成,确保了坳陷带的中北部致密气的大面积封存.结论 认为,不仅前陆盆地隆起带及其邻区、坳陷带的川合—绵阳—黎雅以及冲断带—坳陷带为须家河组致密气发育区,而且须家河组超压体系之上的侏罗系—白垩系都具有致密气勘探的良好前景,上述领域均是未来致密气大气田的重要勘探方向.

In order to discuss the control mechanism of diagenetic systems on the quality of tight sandstone reservoirs， this study analyzed the genetic mechanism of the Xu2 member tight sandstone reservoir in the Zhongtaishan area of the Sichuan Basin by combining data from cores， thin sections， cathode luminescence （CL）， X-ray diffraction （XRD）， scanning electron microscope （SEM）， inclusion analysis， pore permeability and 3-D seismic data. It was found that mainly feldspathic quartz sandstone and lithic quartz sandstone were developed in the Xu2 member of well Zhongtai1 and well Wentan1， respectively， and the reservoir space is mainly composed of feldspars and rock fragment dissolution pores， with porosity 20%. No faulting is evident in well Wentan1； calcite and quartz cementation were mainly developed at the early stage； the diagenetic fluid has a paleosalinity >20%. Siliceous and clay cementation of the dissolution byproducts were evident in both well Zhongtai1 and well Wentan1. These are generally all present and contain intra- or intergranular pores， indicating a short migration distance of the dissolution byproducts. The results show that well Zhongtai1 was in an open diagenetic system during the early stage and in a closed diagenetic system in the late stage， and that well Wentan1 was always in a closed diagenetic system. The dissolution byproducts migrated outwards through fractures to produce a net porosity increment in the open system， which effectively improved the reservoir quality. However， the dissolution byproducts did not migrate outwards in the closed system. They cemented and filled reservoir pores， causing the redistribution of the reservoir pore space. As a result， there is no net increment in reservoir porosity in the closed system， and thus no obvious improvement in reservoir quality. Therefore， it was concluded that reservoir quality in the tight sandstone was controlled by the evolution of the diagenetic system. It was mainly improved in the open system during the early stage， then preserved in the closed system during the late stage. The opening and closure of fractures was the key factor impacting the diagenetic system.

为明确四川盆地上三叠统须家河组致密气成藏要素及有利区带,在重建等时地层与沉积格架的基础上,分析须家河组成藏组合,结合烃源、储层及构造等因素,探讨须家河组致密气藏控藏及富集要素,综合烃源、储层、构造、保存条件、埋深及区内已获有效井地质特征等因素,新建适用于四川盆地须家河组致密砂岩气藏有利勘探区带精细评价的标准体系.研究表明,持续性生烃凹陷、进积型三角洲分支河道砂体以及继承性构造带是须家河组致密砂岩气富集要素,川中—川西过渡带是现阶段四川盆地须家河组致密砂岩气最有利的增储上产地区之一.研究结果对须家河组下一步深入勘探具有重要的指导意义.

探讨川中-川西过渡地区中侏罗统沙溪庙组第二段绿泥石相和溶蚀相储层物性控制因素及孔隙演化过程.通过铸体薄片鉴定、扫描电镜分析、包裹体均一温度以及碳氧同位素等手段,开展了储层压实作用、胶结作用损失孔隙度以及溶蚀作用增加孔隙度的定时-定量研究.结果表明,压实作用是沙二段储层物性变差的第一重要因素,分别造成了溶蚀相和绿泥石相储层28.18％和22.18％的孔隙度损失;由胶结作用造成的孔隙度损失分别为8.25％和9.11％;溶蚀作用是储层物性改善的重要因素,由溶蚀作用所增加的次生孔隙度分别为5.64％ 和1.54％.绿泥石相和溶蚀相储层的孔隙演化有明显差异.绿泥石相储层抗压实能力较强,早期压实损失孔隙较少,在1 km埋深时,孔隙度下降至19.8％;而溶蚀相储层在1 km埋深时,孔隙度下降至16.5％左右.溶蚀作用对二者的影响也存在明显差异,绿泥石相储层增加了1.54％左右的溶蚀孔隙度,在1.7 km埋深时,孔隙度保持在18.5％左右;溶蚀相储层则增加了5.64％左右的溶蚀孔隙度,在1.7 km埋深时,孔隙度为18％ 左右.在晚期胶结物的作用下,储层孔隙度进一步下降,绿泥石相储层在埋深为2.48 km左右时孔隙度下降至12％,直至目前的10.25％;溶蚀相储层在埋深为2.2 km左右时孔隙度下降至12％,现今孔隙度为9.21％.储层致密化均在烃源岩生排烃高峰期之后.

中国致密砂岩气经过10余年的大力发展,目前已成为仅次于美国、加拿大的致密砂岩气生产大国,勘探效果显著,但"如何效益勘探开发"已成为致密砂岩气藏的核心问题,其富集高产主控因素研究迫在眉睫.针对此问题,以四川盆地中部须家河组二段为例,通过对高产气井的解剖,从沉积环境、储层、烃源、构造及成藏等基本地质特征入手分析了富气区高产主控因素,进而探讨天然气高产富集模式.研究取得两方面认识:一是须二段天然气富集高产主要受五大因素控制,且不同区域其侧重点不一.具体表现为古隆起宏观上控制了高产气井的分布,高产气井主要分布在构造圈闭或鼻凸,裂缝是形成高产气井的重要因素:①在隆起带,优质储层对形成高产气井的贡献相对较强;②在斜坡带,烃源的贡献作用更为明显.二是川中地区须二段主要表现为"3种成藏模式、2类油气高产富集模式",具体表现为川中东部合川—营山地区主要为相对优质储层控藏模式、川中西部中台山—秋林地区则为顶部薄储与下伏巨厚的烃源组合模式及川中中部蓬莱—公山庙地区过渡成藏组合.前者富集高产模式表现为构造—优质储层模式,后两者则为储渗体模式.基于这些富集高产因素与模式初步预测了下一步勘探方向及重点勘探领域,以期能指导该区致密气的高效勘探开发.

四川盆地须家河组具有典型的致密砂岩气特征.2006年以来四川盆地大川中地区须家河组致密砂岩先后发现安岳须二段气藏、合川须二段气藏等多个大型岩性气藏,累计提交三级地质储量万亿立方米,勘探成果显著,但气藏投入开发后普遍产水,且水产量较高,累计产气量超过5000×104 m3的高产井较少.2012年,在川中—川西过渡带中台山地区钻探了风险探井ZT1井,在须家河组须二段获高产工业气流.投产后该井日产气量高、产微量水,目前已累计产天然气超1×108 m3.在区域地质研究基础上,通过大量生产井地质条件分析,开展了中台山地区与川中其他地区须二段气藏高产因素差异性分析,明确了优质烃源岩、优质河道砂岩储层、天然裂缝是中台山地区油气富集高产的主控因素,优质烃源岩发育为致密砂岩提供充足气源;优质河道砂岩储层形成高丰度气藏;天然裂缝是油气高产的关键.

Based on the analysis of deposition and tectonism, "residual thickness method" is used to restore the paleokarst landform of Middle Permian Maokou Formation in northwestern Sichuan Basin. With the feature of plain with karst hilllock, the paleokarst landform in this area can be classified into three secondary geomorphic units: karst platform, karst slope and karst groove, in which the karst hillocks and monadnocks on karst platform and karst slope are the favorable zones for the development of karst reservoirs, and favorable exploration zones in the next step. Furthermore, in the karst grooves, the Maokou Formation are often denuded into Members Mao 3 or Mao 2, and the seismic profiles show the top of Maokou Formation in karst groove is missing due to erosion. Members Mao 4 and Mao 3 are generally preserved in the karst platform. The seismic profiles across the karst platform and karst groove show that the NE and NW striking erosion grooves were the result of differential uplift and erosion caused by basement faulting at the end of Middle Permian, which then successively developed and formed the NW striking Guangyuan-Wangcang and the NE striking Jiangyou-Guangyuan troughs in Changxing Period. It is suggested to pay more attention to the geological research and exploration of the shallow carbonate platform areas adjacent to the syneclise and trough in fairly deep water.

The Middle Permian system in the Sichuan Basin represents favorable conditions for the formation of natural gas reservoirs. However, only fracture-vuggy limestone gas reservoirs were discovered in early exploration, whose scales and results were limited. In 2014, a high-yield industrial gas flow was produced there in Well Shuangtan 1, the major risk-exploration well, which means a great breakthrough in natural gas exploration in the Middle Permian system, Sichuan Basin. This plays a strategically important and guiding role in oil and gas exploration in ultra-deep marine formations. This paper illustrates the deployment background of Well Shuangtan 1, major exploration achievements and its significance in the gas exploration in the Middle Permian system, Sichuan Basin. Moreover, exploration potential and subsequent exploration targets of Middle Permian are analyzed in terms of its hydrocarbon source conditions, sedimentary characteristics, reservoir characteristics and areal extension, resources and favorable exploration zones. It is concluded that the Middle Permian system in the Sichuan Basin is a continent-connected carbonate platform with carbonate deposits being dominant. There are three types of reservoirs, including pore dolomite, karst fracture-vuggy, and fracture, which present considerable natural gas resources and huge exploration potential. Favorable exploration zones classification of Middle Permian in the Sichuan Basin indicates that the most favorable exploration zones are mainly distributed in the central-western Sichuan Basin. In particular, the northwestern Sichuan Basin is the most favorable exploration area which is characterized by the most excellent hydrocarbon source conditions, developed dolomite reservoirs and zonal distribution of structures.

上三叠统须家河组二段是四川盆地重点勘探层系之一.通过200佘口井资料分析,对比了不同类型砂体储集性能后认为:优质储层岩石类型为浅灰色中—细粒长石岩屑砂岩,孔隙类型为粒间孔和粒内溶孔,主要发育绿泥石胶结,具有低电阻、低伽玛、高声波的“二低一高”电性特征;沉积相和成岩相控制了优质储层的发育,水下分流主河道微相是优质储层形成的关键,绿泥石胶结有利于原生孔隙的保存.结合测井和地震储层预测,明确了川中地区须二段优质储层为大面积叠置连片分布,指出了有利勘探区带.成果对川中地区须家河组二段勘探整体部署有重要意义.

利用岩心薄片、岩屑薄片、测井和地震资料对四川盆地须家河组砂岩类型开展研究,明确了川西至川中地区须一、须三、须五段内大面积发育灰屑砂岩,属于三角洲前缘水下分流河道沉积,具有成熟度低、灰质胶结重、粒间孔少、物性差等特征,局部发育相对高孔“甜点”砂体.电性上表现为“三低两高”特征,即低自然伽玛、低声波时差、低补偿中子、高电阻、高密度;地震上表现为高阻抗特征.以灰屑砂岩作为典型标志层,提出在剑阁—蓬莱地区发育大型三角洲体.该三角洲体具有继承性发育特征,在须一、须三、须五段沉积时期,与优质泥质烃源岩共生,生储盖组合好,具备形成大规模致密气的成藏条件.图6参4

A detailed study of the origin and evolution of the Sichuan Basin is of great significance to further exploration and development in this basin.Based on previous study results,it is proposed that the western Sichuan and Dabashan foreland basins are combined into one basin generally named the Sichuan Foreland Basin.Four major evidences are presented to support this viewpoint : the same tectonic settings,the same impact of forming mechanism on the foreland basin,the unity of " appearance " and " nature ",and the same foreland bulge.Particularly,we focused on the analysis of the regional tectonic settings and evolution process of this basin as well as the relationship between the prototype basin and major structural zones.The results show that the Sichuan Foreland Basin has experienced a complex evolution process controlled by different orogenic belts in different geologic periods,during which the extent of sedimentation changed greatly and the depocenter migrated frequently.The vertical overlapping of different types of basins and lateral superimposition of prototype basins of different ages resulted in the Sichuan Foreland Basin under the control of multiple orogenic belts.As a result,6first-order and 15second-order structural zones are identified.The 6first-order structural zones include nappe structural zone at the trailing edge of orogenic belt,exposed front overthrust structural zone at the front edge of orogenic belt,piedmont blind front structural zone,central depressed zone of foreland basin,front slope zone of foreland basin,and front uplifted zone of foreland basin.

四川盆地上三叠统须家河组天然气勘探在前陆盆地及岩性油气藏理论指导下,进展显著,先后寻找到广安、合川、安岳等大型岩性气藏,表明具备在前陆背景下形成大气区的条件.为此,在原型盆地恢复的基础上,综合研究须家河组的成藏背景、成藏地质条件、油气富集特征及油气藏类型分布等要素,分析了须家河组前陆大气区的基本特征、气区边界条件及空间分布特征、成藏标志及资源标志.结论指出:须家河组前陆大气区资源潜力大,勘探目的层除须二、四、六段的砂岩外,须一、三、五段在盆地局部地区也具勘探前景,具有多层段含气、近源聚集、多种气藏类型并存的特征,勘探应以寻找岩性气藏为主.同时还认为,四川前陆盆地在前陆隆起带天然气勘探已取得重要勘探成果的基础上,前陆斜坡带是近期的重要勘探领域,前陆坳陷区是今后的重点勘探区.

Taking the compacted sandstone gas accumulation in Upper Triassic Xujiahe Formation of the middle area of Sichuan as example,this paper discusses the process and mechanism of the compacted sandstone gas accumulation and provides the evidences for the next pace of exploration and exploitation of this sandstone gas accumulation.Through simulating the hydrocarbon-generating history of the typical structure in the middle area of Sichuan,it can be concluded that the generation process of the source rocks in Xujiahe Formation with different structures have some similarities as a whole.By the method of fluid inclusions' homogeneous temperature,K-Ar dating method of authigenic minerals and ESR dating method of crack filling minerals,the three periods of the gas accumulation are determined in Xujiahe Formation of the middle area of Sichuan.The former two periods were mature and highly mature hydrocarbons filling into the reservoir in Jurassic,and the last period had been the structure's uplifting and adjusting period since Late Cretaceous,which transformed the early natural gas reservoir and then made it accumulated again.By and large,the compacted sandstone gas has three characteristics in Xujiahe Formation of Middle Sichuan area.First,the migration distance of the natural gas is short,second,the filling form of the natural gas is in free phase,and thirdly,the decompression and inflation of the natural gas plays an essential role in the natural gas accumulation in the process of uplift.

With the help of theories about foreland basins and litholgic reservoirs, a great success has been made in the natural gas exploration in the upper Triassic Xujiahe Formation of the Sichuan Basin. For example, large lithologic gas reservoirs in such areas as Guangan, Hechuan and Anyue were discovered successively, which indicates that it is possible for the formation of large gas play fairways in foreland settings. In view of this, based on the restoration of prototype basins, this paper comprehensively studies the settings and geological conditions of gas accumulation, features of gas enrichment as well as types and distribution of gas reservoirs in the Xujiahe Formation, and analyzes the basic features, boundary conditions and spatial distribution as well as gas accumulation marks of the large gas play fairway in the Xujiahe Formation in the foreland area, where there is a huge gas resource volume. The exploration targets including the 2 nd, 4 th and 6 th members of the Xujiahe Formation have large exploration potential. In addition, the 1 nd, 3 rd and 5 th members are also prospective in local areas. The gas reservoirs in the study area are featured by multiple gas-bearing intervals, near-source accumulation and multiple reservoir types, and exploration should focus on lithologic gas reservoirs. In addition, based on the achieved great results from the exploration in the uplifted zone of this foreland basin, it is pointed out that the slope zone will be the next important exploration target at present and the depressed zone will be the target in near future in this foreland basin.

The reservoir-formimg characteristics of Xujiahe Formation in Sichuan Basin are stated in this paper,and also,it is pointed out that the formation is a set of coal- bearing clastic rack strata formed on the base of marine facies carmonate platform in the western margin of upper Yantz platform,it is one of the most prospective formations in Sichuan Basin due to the exclusive hydrocarbon generation,reservoir and cap association.Vertically,in the Xujiahe Formation gas reservoirs,three sets of sandwich-like reservoir-forming associations develop,which takes lithologic gas reservoir,structural-lithologic combination gas reservoir as the main existing way,and is of the characteristics of multiple reservoir-forming associations,high water saturation,complicated gas and water eolation,diverstified gas reservoir types,and controlled by hydrocarbon source,reservoirs,ancient and present structures and foreland basin texture.The research indicates the reservoir-forming conditions and the matching of Xujiahe Formation in Sichuan Basin are generally good.There exist five favorable gas accumulation areas with bright explorative prospect.