The Feixianguan Formation reservoir rocks in Northeast Sichuan are mainly composed of dolostones. Through investigation it was considered that the dolomites in Feixianguan Formation may be divided into three types: the first is the hypautomorphicxenomorphic finemedium crystalline grain dolomite; the second is the dolomicrite and the third is the automorphic mediumcoarse crystalline grain dolomite. They have different geochemical characteristics in the degree of order, the carbon and oxygen isotopes, the microelement, the inclusion, the cathodoluminescence, and so on. The geneses of these three types of dolomites are mixed water dolomitization, seepage reflux dolomitization and burial dolomitization respectively, in which the mixed water dolomitization was one of principal diagenetic factors causing the excellent oolitic beach reservoir being well developed in the region. The beneficial mixed water dolomitization zones mainly occurred in the oolitic bar (beach) along platform edge, of which the distribulation area is roughly identical with that of gypsum, its petrophysical property being slightly poor. (Financed by the project of the Key Technologies Research and Development Program of the Tenth FiveYear Plan, No 2001BA605A-04-03)

川东北地区飞仙关组储集岩主要是白云岩。白云石化成因类型对勘探有利区块和勘探目标的选择都具有重要的意义。研究认为，区内飞仙关组岩石中的白云石主要有三种类型：第一种为半自形—他形细—中晶白云石；第二种为泥晶白云石；第三种为自形中晶-粗晶白云石。它们在有序度,C、O同位素,微量元素,包裹体,阴极发光等方面具有不同的地球化学特征。通过研究认为，这三种类型的白云石成因分别为混合水白云石化、渗透回流白云石化和埋藏白云石化。其中混合水白云石化是该区优质鲕滩储层得以发育的主要成岩因素之一，有利的混合水白云石化带主要发生在台地边缘鲕坝（滩）中，这也是鲕滩储层发育的最有利地区，渗透回流白云石化主要发生在台地边缘鲕粒坝（滩）后方的局限海环境中，其分布范围大致与石膏分布范围一致，储集物性略差。

川东飞仙关组白云岩包括微晶白云岩、具原始结构的粒屑白云岩和结晶白云岩等3种主要的结构类型，其中结晶白云岩是重要的天然气储集岩。根据白云岩的锶含量和锶同位素组成，结合相关岩石学和地球化学资料，本文研究了白云石化过程中锶的迁移和相应的同位素变化。三种类型白云岩具有类似的锶含量，但其^87Sr/^86Sr比值均不同程度地高于同期海水值，锶在白云石和方解石（或文石）中分配系数的差别是影响白云石化过程中白云石锶含量最为重要的因素。不同流体中锶的混合方程计算表明：如果白云石化流体是大气淡水和海水的混合流体，则结晶白云岩的白云石化流体相当于94％的大气淡水和6％的海水混合，具原始结构的粒屑白云岩的白云石化流体相当于81％的大气淡水和19％的海水混合，微晶白云岩的白云石化流体相当于92％的大气淡水和8％的海水混合。但极低的锰含量、相对较高的锶含量和包裹体均一化温度等资料均不支持有94％的大气淡水参与了结晶白云岩的白云石化作用，其白云石化流体应是埋藏成岩过程中囚禁的近同期、但是非同期的海源流体，其来源可能与埋藏成岩过程中该时间段蒸发盐的溶解有关。锰含量、锶含量、^87Sr/^86Sr比值都表明具原始结构的粒屑白云岩和微晶白云岩的白云石化过程都存在大气淡水的介入，微晶白云岩的白云石化可能与潮坪环境的蒸发泵机理有关；具原始结构的粒屑白云岩的白云石化可能与混合水白云石化作用有关，这类白云岩分布于向上浅滩化旋回滩体的顶部，这是埋藏前或浅埋藏阶段大气淡水与海水混合白云石化作用最容易发生地带。

A shallow marine carbonate rock sequence with shale and evaporate developed in the Lower Triassic Feixianguan Formation in northeastern Sichuan Province.The edge of carbonate platform commonly developed some oolitic banks，which are dolomitized.The Feixianguan Formation which are formed by oolitic bank dolostone is the most important gas reservoir in northeastern Sichuan Province. Some researchers considered them as the origin of mixing for fresh water and sea water，the others make up them into model for dolomitization by seepage-reflux.The Feixianguan oolitic bank dolostone included stable isotopic oxygen composition of -6.73‰～-3.65‰(PDB)（-10.81‰ in Luojiazhai area），the average is -4.89‰(PDB)，stable isotopic carbon composition of +0.57‰～+3.00‰(PDB)，for matrix and vug-filling saddle dolomite and bright dolomite cement，stable isotopic strontium composition of 0.70735～0.70800 for 87 Sr/ 86 Sr rate.These data about oolitic bank dolostone indicate that the dolomitization was formed in the buried condition.After measuring fluid inclusion homogenization temperature，dolomitization temperature and isotopic oxygen of dolomite are known，and then the isotopic oxygen composition of dolomitizing fluid can be calculated( δ 18 O dol - δ 18 O water =［3.2×10 6 T  -2 ］-1.5，from Friedman and ONeil (1977)). The average is about +4‰ (SMOW).According to the positive covariance between fluid isotopic oxygen and carbon composition and fluid salinity during evaporation of seawater，fluid δ D can be calculated.It is about +25‰(SMOW).The measurement of fluid inclusion salinity suggest that dolomitizing fluid is a brines，which is about several times of seawater.Dolomitizing temperature changes from 90 ℃ to 130 ℃.As far as our knowledge goes，stable isotopic O and C composition of the Lower Triassic oolitic bank dolostone is similar to these of the uppermost Permian reef dolostone.Therefore，it is possible that their dolomitizing fluids are identified source.However，the reason for this conclusion needs to be perfected in the future.

川东北地区下三叠统飞仙关组为浅海碳酸盐岩夹泥页岩与蒸发岩序列，而在碳酸盐岩台地边缘通常发育一些白云石化的鲕粒滩。这些鲕粒滩白云岩储集层是川东北地区主要的产气层，一些学者认为该套白云岩为大气淡水与海水的混合水白云石化成因，另外一些学者将其视为回流—渗透白云石化成因。飞仙关组鲕粒滩白云岩稳定同位素氧值一般为-6.73‰～-3.65‰(PDB)，平均值为-4.89‰(PDB)（罗家寨地区为-10.81‰(PDB)），稳定同位素碳值一般为+0.57‰～+3.00‰(PDB)。对基质和孔洞中充填的鞍状白云石和亮晶白云石胶结物而言，稳定同位素 87 Sr/ 86 Sr值为0.70735～0.70800。这些有关鲕粒滩白云岩的数据表明白云石化作用是在埋藏条件下进行的。在测定流体包裹体的均一化温度后，计算出白云石化流体稳定同位素氧成分( δ 18 O 白云石 - δ 18 O 水 =［3.2×10 6 T  -2 ］-1.5，来自 Friedman 和 ONeil（1977）)，其平均值约为+4‰(SMOW)。根据流体稳定同位素氧、碳成分与海水蒸发时流体盐度的正相关性，计算出流体 δ D平均值约为+25‰(SMOW)。流体包裹体盐度测定表明，白云石化流体是一种超盐度卤水，其盐度是海水的数倍，白云石化的温度为90～130℃。由于下三叠统鲕粒滩白云岩的稳定同位素氧和碳成分与上二叠统生物礁白云岩的稳定同位素氧和碳成分类似，因此，它们的白云石化流体很可能是同一来源。然而，这一结论还有待于进一步研究。

It is thought generally that dissolution of acidic fluids is an important controlling factor for forming carbonate reservoir. This study focuses on the ability of dissolution of different type carbonate rocks in CO2 solution using a new simulating experiment way. The results show that the ability of dissolution of different carbonate rocks changes from low to high and again to lower with the simulating temperature from 25℃ to 200℃,and the maximum ability of dissolution is at 60～90℃; the ability of dissolution of dolomite is lower compared with the ability of dissolution of calcite, and the ability of dissolution of transitional type carbonate rocks is between calcite’s and dolomite’s, and the ability of dissolution is very lower and their different is very small for all type carbonate rocks in CO2 solution when the simulating temperature is higher than 150℃.This indicates that CO2 solution has important effect on dissolution of limestone and less effect on dissolution of dolomite during late stage of early diagenetic to early stage of middle diagenetic and the forming of good dolomite reservoir is related to dolomitization.

普遍认为酸性流体的溶蚀作用是碳酸盐岩储层形成的重要制约因素。本文以一个全新的模拟实验方式对不同类型碳酸盐岩在CO2水溶液中的相对溶蚀能力进行了研究，结果发现随温度从常温至200℃，碳酸盐岩的溶蚀能力由弱变强再变弱，在60～90℃区间内溶蚀能力最强。白云岩不管在低温还在高温环境下，总比灰岩更难溶蚀，过渡类型的岩类介于二者之间，当温度大于150℃后，CO2对碳酸盐岩的溶蚀能力变得越来越弱。灰岩与白云岩的溶蚀差异也变得越来越小。这暗示碳酸盐岩在早成岩晚期—中成岩早期，CO2水溶液对灰岩的溶蚀作用有重要影响，而对白云岩的溶蚀作用影响较小，白云岩优质储层的形成可能与碳酸盐岩中钙质的流失或白云岩化作用有关。

The carbonate platform and trough facies of the Late Permian to the Early Triassic Feixianguan Formation are outcropped in the east Fankuai in Xuanhan County, northeastern  Sichuan Basin. The distance between the Jichang carbonate slope facies section which separates the platform facies and trough facies， and the platform margin facies section is less than one kilometer. According to the characteristics of the outcrops of carbonate slope facies，  the carbonate slope facies can be identified on the seismic sections of KaijiangLiangping trough in northern Sichuan Basin and the shallow water carbonate sedimentary area and deep water carbonate sedimentary area was subdivided. Slope facies is characterized by the obvious inclined reflection. High amplitude reflection caused by the mudstone of Dalong Formation and the bottom of the Feixianguan Formation of the trough facies onlapped towards the platform and disappeared. The twoway travel time of trough facies in the Upper Permian is less than 100 ms and is far less than that of the Feixianguan Formation， while the twoway travel time of platform facies is more than 150 ms which is similar to or slightly longer than that of the Feixianguan Formation. The incline reflection characteristics of slope facies,  indicate that the carbonate slope of KaijiangLiangping trough was developed in the middle Late Permian and migrated to the carbonate platform with transgression occurred. It became the steepest slope when the transgression reached the maxium at the end of the Late Permian. In the Feixianguan Period， slope gradient decreased with the sea level fall and the slope migrated to the trough. At the end of the Late Permian， the slope gradient of Liangping area which is the south of the trough was around 3°and slope became steep towards the northeast and northwest. In the Xuanhan area， the slope gradient was about 20°， and it achieved the highest gradient of  40° in Yilong area. The trough was a basin which became deeper to wards the north. According to the height of the inclined reflection of slope in the Late Permian， it is calculated that the depth of the KaijiangLiangping trough was around 350 to 450 meters. The slopes are characterized by obvious progradation of the Feixianguan Formation in Cangxi， Yilong and Liangping areas and the slope is dominated by aggradation in Xuanhan area. The currently drilled thickest oolitic beach reservoir is located in the platform margin adjacent to the aggradational slope in Xuanhan area. The thickness oolitic dolomite reservoir in individual wells is larger than 300 meters.

　四川盆地东北部宣汉县樊哙以东地区晚二叠世—早三叠世飞仙关期碳酸盐岩台地相—海槽相剖面均出露地表。分隔台地相和海槽相的碳酸盐岩斜坡相的鸡唱剖面距台地边缘相剖面不足１ｋｍ。根据地面露头碳酸盐岩斜坡相带的特征，可在四川盆地北部环开江—梁平海槽的地震反射剖面上有效地识别出碳酸盐岩斜坡相，并划分出浅水、深水碳酸盐沉积区。斜坡相具有明显的倾斜反射，海槽相区大隆组及飞仙关组底部泥岩的强振幅反射在斜坡下部向台地上超并消失。深水相区上二叠统地震波双程反射时间都小于１００ｍｓ且明显短于飞仙关组，而台地相区上二叠统双程反射时间多大于１５０ｍｓ，且与上覆飞仙关组地震波反射时间相近或略长。碳酸盐岩斜坡相倾斜反射的特征表明环海槽的碳酸盐岩斜坡开始发育于晚二叠世中期，并随海侵向碳酸盐岩台地方向迁移，在晚二叠世末期海侵达到顶峰时形成最陡峭的斜坡。飞仙关期斜坡随海平面缓慢下降而逐渐向海槽方向迁移并变缓。晚二叠世末期海槽南端梁平地区斜坡坡度在３°左右，向北东、北西方向变陡，宣汉地区约２０°，苍溪、仪陇地区最高达４０°以上，海槽呈向北逐渐加深的箕形。根据晚二叠世末期斜坡倾斜反射高度计算，开江—梁平海槽的水深在３５０～４５０ｍ左右。苍溪、仪陇及梁平地区飞仙关组斜坡进积明显，宣汉地区的斜坡以加积为主。目前钻遇的最厚鲕滩储集层位于宣汉加积斜坡附近的台地边缘，单井的鲕粒白云岩储集层厚度超过３００ｍ。

Kaijiang-Liangping trough existed for 700 My.controlled the deposition of Feixianguan Formation of lower Triassic in the northern Sichuan Basin. In the short period of the trough existed, relativelysymmetrical carbonate rock sedimentary system that be analogous to Wilson facies pattern could be formed in its both sides. Through researching on petrology data of detailled drilling, outcrop and so on, Feixianguan Formation mainly developed six types of sedimentary facies including evaporate platform, restricted platform, open-platform, platform marginbeach, slope and basin facies. The basin facies that was shallower than deep-sea basin which mainly developed dark marl and lime mudstone; the depositonal characteristics of the slope facies are distinct, but shelf facies poor developed which may be for the sake of the depth of the trough, because it is so small; with the gradual close of the trough, seawater became shallow, the characteristics of the depositional evolution from basin facies to slope, platform margin beach, open-platform, restricted platform and evaporate platform within the trough are dinstinct; to the top of the Member 4 of Feixianguan Formation, all of the Northern Sichuan basin were deposited equally by restricted platform facies or evaporate platform facies. The period of the trough existed is so short that the course of evolution of sedimentary facies is quite clear in plane and longitude. With the gradual close of the trough, seawater receded toward to open sea, platform margin oolitic beaches moved toward to trough too, that was open sea, growing horizons of them rised gradually at the same time.

川北地区下三叠统飞仙关组沉积主要受开江梁平海槽控制,短期存在的海槽在其两侧形成了较为对称的类似于威尔逊相模式的碳酸盐岩沉积体系。通过详细的钻井、露头等岩石学资料的研究,认为飞仙关组主要发育蒸发台地、局限台地、开阔台地、台地边缘滩、斜坡和盆地相共六类沉积相。其盆地相较深海盆地相浅,主要发育深色泥灰岩和灰质泥岩;斜坡沉积特征明显,见浊流、碎屑流沉积,但陆棚相不发育,可能是海槽深度小、面积小引起的;随着海槽的关闭,海水变浅,海槽内部的沉积由盆地相向斜坡、台地边缘滩、开阔台地、局限台地、蒸发台地相演化特征明显;到飞四段顶部,整个川北地区均一化为局限台地相或蒸发台地相沉积。由于海槽存在时间短,沉积相在平面上和纵向上的演化过程十分清晰。随着海槽的关闭,海水向广海方向退却,台缘鲕滩也向海槽边界,即向广海方向发生了迁移,而且其发育层位逐渐变高。

Mixed water dolomization is the basis of platform fringing oolitic beach reservoir of Feixianguan group. With the petrologic analysis of dolomite, the geochemistry analysis as well as the speculation of temperature and depth, it is proved that dolomization came into being in mixed water. The authors summarize the dolomization genetic model of oolitic beach reservoir. The mixed water dolomization is mainly controlled by frequent change of the sea level.

残余鲕粒白云岩是飞仙关组鲕滩气藏最有利的储层。白云石化作用是有效储层形成的关键因素之一。通过对白云岩进行岩石学特征分析、地化特征分析及成岩温度和深度的研究,认为白云石化作用发生在混合水成岩环境,由混合水白云石化机理产生。海平面频繁的变化是发生混合水白云石化作用的主要动力。总结了该储层发生混合水白云石化作用的模式。

Based on the depositional features, pore types, and geochemical behaviors of pore filler, two stages of burial dissolution are identified in the Feixianguan Formation reservoirs in Northeast Sichuan Basin. The first stage was related with the organic acid generated by the mature Upper Permian source rocks of trough facies, while the second stage was related with the H 2 S generated from thermochemical sulfate reduction reaction. The dissolution mainly occurred in the oolitic beach facies at platform margin, while it was weak in the lagoon and point bar facies in platform interior. Analysis shows that the platform margin is favorable for gas exploration, in contrast to the unfavorable platform interior.

根据沉积特征、孔隙类型及孔隙充填物的地球化学特征认识到川东北地区飞仙关组储层中有2期埋藏溶蚀作用：第一期埋藏溶蚀作用与上二叠统海槽相烃源岩成熟产生的有机酸有关；第二期埋藏溶蚀作用与热化学硫酸盐还原反应产生的H 2 S有关。两期埋藏溶蚀作用主要发生在台地边缘鲕粒滩相中，台地内泻湖及点滩相中溶蚀作用则相对较弱。分析认为：台地边缘成为天然气勘探的有利地区，台地内为欠有利地区。