川东北元坝地区须三段钙屑砂砾岩储层特征及控制因素

杜红权，王　威，周　霞，郝景宇，印　峰

(中国石化 勘探分公司，四川 成都　610041)



川东北元坝地区须三段钙屑砂砾岩储层特征及控制因素

杜红权，王威，周霞，郝景宇，印峰

(中国石化 勘探分公司，四川 成都610041)

四川盆地元坝地区须家河组三段多口井测试获得中-高产工业气流，具有良好勘探开发潜力，但其储层特征及控制因素不明确。利用岩心、薄片及分析化验资料对研究区储层岩石类型、储集空间、物性及发育控制因素进行综合研究。研究认为：储层岩石类型以钙屑砂岩和灰质砾岩为主，具有“石英、长石含量低，碳酸盐岩屑(碳酸盐砾)含量高”的特征。储集空间以粒间溶孔、粒内溶孔及裂缝为主，钙屑砂岩与灰质砾岩储层差异明显。钙屑砂岩为裂缝-孔隙型储层，而灰质砾岩为孔隙-裂缝型储层，物性总体较差，具有低孔、低渗的特征。储层发育主要受沉积微相、岩石类型、溶蚀作用及裂缝共同控制。有利沉积微相及岩石类型是优质储层发育的基础，溶蚀作用及裂缝是优质储层发育的关键。水下分流主河道中-粗粒钙屑砂岩为最有利岩相，高含量钙屑组分在后期溶蚀作用及裂缝改造下易形成优质储层。

钙屑砂砾岩储层；储层特征； 须家河组三段；元坝地区；四川盆地

近年来随着勘探投入加大，元坝地区须家河组天然气勘探取得重大突破，发现了多个岩性或构造-岩性复合气藏，多口井在须家河组三段(须三段)钙屑砂砾岩测试获中-高产工业气流，显示出须三段具有良好的勘探潜力，进一步拓宽了元坝地区陆相油气勘探领域。然而须三段储层发育控制因素尚缺乏清晰的认识，制约了须三段储层综合评价及勘探部署。借鉴前人研究成果认识[1-8]，综合大量岩心、薄片资料及多种分析化验资料详细分析了钙屑砂砾岩储层特征、探讨了储层形成机制及发育主控因素，以期对其它地区钙屑砂岩勘探具有借鉴意义。

1　区域地质背景

元坝地区位于四川盆地川北低缓褶皱带北缘，北邻米仓-大巴前陆冲断带，东接川东高陡断褶带，西为龙门山逆冲推覆带[9-10](图1)。印支运动以来，在松潘-甘孜、秦岭造山带与四川盆地之间，发育了北东向的龙门山和北西向的米仓山-大巴山两个巨型推覆构造带，形成了造山带-盆缘推覆山系-盆地组合格局[11]。在这一复杂的组合格局中，四川盆地中、新生代陆相地层沉积厚度均在4 000 m以上，蕴藏着丰富的石油和天然气资源。

元坝地区须三段物源主要来自龙门山北段摩天岭古陆二叠系-三叠系碳酸盐岩发育区[12-14]，沉积物供应充足，砂砾岩分布广、延伸远，自北西向南东粒度变细、碳酸岩岩屑含量降低。研究区须三段主要发育浅水缓坡型辫状河三角洲-湖泊沉积体系[15-17](图2)，元坝西北部辫状河道及水下分流河道微相叠置连片分布。元坝地区须三段岩石类型特殊，发育多套厚层、分布稳定的钙屑砂岩或灰质砾岩(统称为钙屑砂砾岩)

沉积，电性特征具有明显的中-低自然伽马(25～60 API)、相对高声波时差(49～61 μs/ft)、相对低密度(2.65～2.8 g/cm3)、高电阻率(300～8 000 Ω·m)特征，主要分布在须三上、中亚段，垂向上叠置，横向上连续，自北西向南东厚度减薄、粒度变细，平面上大面积分布。

2　储层特征

2.1岩石学特征

元坝地区须三段储层岩石类型复杂，主要为钙屑砂岩和灰质砾岩。

1) 钙屑砂岩

钙屑砂岩是指碳酸盐岩岩屑含量大于50%的岩屑砂岩(图3a,b)，具有极低的GR值和极高的RT值，分选中等-好，磨圆次棱角状-次圆状，颗粒支撑结构，多为孔隙式胶结。

碎屑成分中石英含量分布在4.0%～53.0%，平均为27.0%，长石含量分布在0.5%～9.0%，平均为2.3%，岩屑含量分布在74.0%～96.0%，平均为77.0%，岩屑成分以碳酸盐岩岩屑为主(表1)。

填隙物成分一般为沥青质、粘土杂基和钙质、硅质等胶结物，杂基含量一般在0.2%～6.0%，平均为3.7%，胶结物含量在0.2%～30.0%，平均为13.5%，成分以方解石和白云石为主，含少量硅质和菱铁矿。

图1　元坝地区构造位置Fig.1　Structural location of Yuanba area

图2　元坝地区须三段沉积层序特征Fig.2　Sedimentary sequence characteristics of the 3rd Member of Xujiahe Formation in Yuanba area

2) 灰质砾岩

砾石成分以碳酸盐岩砾石为主(图3c，d)，分选中等，磨圆以次圆状为主，粒径一般为10～60 mm，颗粒支撑结构，砾石成分以灰岩、白云岩为主，偶见鲕粒、有孔虫等生屑颗粒，颗粒间多被石英、钙屑砂岩等颗粒及方解石胶结物充填。

2.2储集空间类型

依据铸体薄片资料，元坝地区须三段储层储集空间以粒间溶孔、粒内溶孔及裂缝为主，少量残余原生粒间孔及杂基微孔。不同类型储层储集空间类型不同，钙屑砂岩储层储集空间以粒间溶孔为主(图3e)，含少量粒内溶孔(图3f)，偶见原生粒间孔，而灰质砾岩储层微裂缝发育(图3g，h)，且类型多样，主要有砾缘缝、穿砾缝等。

2.3物性特征

研究区须三段116个样品统计可知，储层岩心孔隙度最大值为7.16%，最小值为1.39%，平均值为2.92%，其中孔隙度为2%～3%的样品占总样的57.7%，孔隙度为3%～4%的样品占总样的25.9%(图4a)。渗透率最大值为363.213 6×10-3μm2，最小值为0.001 4×10-3μm2，几何平均值为0.013 5×10-3μm2。其中渗透率为(0.005～0.01)×10-3μm2的样品占总样32.7%，渗透率为(0.01～1)×10-3μm2的样品占总样39.7%(图4b)。总体上，研究区须三段钙屑砂砾岩储层物性较差，具有低孔、低渗的特征。

图3　元坝地区须三段储层储集空间特征Fig.3　Characteristics of reservoir space of the 3rd Member of Xujiahe Formation in Yuanba areaa. YL22井,埋深4 158.46～4 158.73 m,须三中亚段，钙屑砂岩;b. YL12井,埋深4 325.00 m，钙屑砂岩;c. YL20井，埋深4 146.59～4 146.82 m,须三中亚段,灰质砾岩;d. YL10井，埋深4 186.00 m，灰质砾岩;e. YB2井，埋深4 378.50 m，粒间溶孔;f. YL15井，埋深4 275.66 m，粒内溶孔;g. YB6井，埋深4 299.46 m，砾缘缝;h. YL15井，埋深4 282.43 m，穿砾缝;i.旺苍剖面须三段，碳酸盐岩屑溶解形成铸模孔;j.旺苍剖面须三段，碳酸盐岩屑溶解形成铸模孔;k. YL18井，埋深4 528.94 m，碳酸盐岩屑砂岩中发育的裂缝，裂缝穿过白云岩岩屑、灰岩岩屑、方解石胶结物;　　　l. YL18井，埋深4 528.44 m，裂缝位于灰岩砾石和填充物(由碳酸盐岩屑砂岩组成)的连接处，裂缝穿过白云岩岩屑表1　元坝地区钙屑砂岩岩石组分含量统计Table 1　Statistics of rock constituents of the calcareous sandstone in Yuanba area

数值石英/%长石/%岩屑/%岩浆岩屑变质岩屑沉积岩屑杂基/%胶结物/%方解石白云石硅质最小值40.50.21.073.00.21.03.00.2最大值539.02.02.094.06.025.05.01.0平均值272.31.01.576.23.712.83.50.7

图4　元坝地区钙屑砂砾岩储层孔隙度(a)、渗透率(b)直方图Fig.4　Histograms showing porosity(a)and permeability(b)of the calcareous coarse clastic reservoirs in Yuanba area

依据储层段岩心样品物性分析，钙屑砂岩储层孔隙度最小值为1.56%，最大值为7.16%，平均值为3.0%，而灰质砾岩储层孔隙度最小值为1.39%，最大值为3.73%，平均值为2.51%。钙屑砂岩储层渗透率最小值为0.001 4×10-3μm2，最大值为2.004 8×10-3μm2，平均值为0.008 1×10-3μm2，而灰质砾岩储层渗透率最小值为0.003 4×10-3μm2，最大值为363.213 6×10-3μm2，平均值为0.210 3×10-3μm2。从孔渗关系图看，钙屑砂岩孔渗具有明显的正相关性，而灰质砾岩孔渗相关性差，明显受裂缝影响(图5)。总体上，研究区须三段钙屑砂岩与灰质砾岩储层物性差异明显，钙屑砂岩储层孔隙性好、渗透性差，为裂缝-孔隙型储层，而灰质砾岩储层孔隙性差、渗透性好，为孔隙-裂缝型储层。

3　 控制因素

3.1有利沉积微相及岩石类型是优质储层发育的基础

元坝地区须三段主要发育浅水缓坡型辫状河三角洲-滨浅湖沉积体系，广泛发育辫状河道、水下分流河道及河口坝等沉积微相。元坝地区须三段储层沉积微相、岩石类型、物性及测试产能对比分析表明(表2)，水下分流主河道中-粗粒钙屑砂岩及辫状主河道灰质砾岩储层物性好、测试产能高，是最有利储层，水下分流次河道及河口坝细粒钙屑砂岩储层为较有利储层。水下分流主河道及辫状主河道微相水动力强，沉积物以灰质砾岩及中-粗粒钙屑砂岩为主，钙屑组分含量高，成岩期受破裂改造及溶蚀作用影响易形成优质储层，而河道前缘河口坝、席状砂及河道边缘因水动力条件弱，钙屑砂岩粒度变细，钙屑含量降低，分选差，受压实作用影响砂岩致密且不易改造形成储层[18-21]。从试采情况看，水下分流主河道中-粗粒钙屑砂岩储层试采效果好，YL12井须三段钙屑砂砾岩储层当前日产气为7.6×104m3，油压为13 MPa，累计产气为4 576×104m3。YL7井须三段中-粗糙钙屑砂岩储层当前日产气为4.3×104m3，油压为8.9 MPa，累计产气为4 563×104m3，进一步证实水下分流主河道微相发育的中-粗粒钙屑砂岩是元坝须三段相对优质储层发育的有利岩相。

3.2钙屑溶蚀作用及裂缝改造是优质储层发育的关键

受龙门山造山带影响，研究区须三段沉积时期盆地西北部隆起抬升剧烈，碳酸盐岩地层遭受剥蚀，物质供给充足，沉积物以钙屑砂砾岩为主。受水动力条件、介质搬运及沉积相带等影响，靠近物源区，钙屑组分含量高且钙屑砂砾岩厚度大。钙屑砂砾岩储层经历了复杂而强烈的成岩变化，强烈的压实作用和胶结作用使碎屑颗粒多为线—凹凸接触，粒间孔被大量细粒组分及钙质胶结物充填，连通性差，普遍具有低孔、低渗、致密的特征。须三段为富含水生及陆生植物的煤系地层，随着埋深的增加，有机质开始成熟，干酪根热解形成大量有机酸，且深层干酪根热裂解释放出来的CO2与水作用形成碳酸。这种酸性水或有机酸随泥岩压实进入相邻的钙屑砂砾岩中，对其中的钙屑组分进行溶蚀形成碳酸盐胶结物，同时伴随大量粒间溶孔、粒内溶孔，改善砂体储集性能[22-26](图3i，j)。研究区须三段碳酸盐岩屑和碳酸盐胶结物含量呈负相关关系，碳酸盐岩屑在深埋背景的压力作用下溶解，使得钙、镁元素转入溶液，引起物质再分配，造成在低压处(孔隙空间处)碳酸盐矿物的沉淀(图6a)及溶蚀孔隙的形成(图6b)。另外，由于差异压实和构造破裂作用所形成的裂缝叠加进一步改善了该类储层的渗透性(图3k，l)，同时也可促进富含有机酸和无机酸的孔隙流体沿裂缝流动，形成溶蚀孔缝，使孔隙间的连通性变好。裂缝是碳酸盐岩屑砂岩有利储层发育和天然气富集高产的另一重要因素。

图5　元坝地区钙屑砂砾岩储层孔、渗关系Fig.5　Relationship between porosity and permeability of the calcareous coarse clastic reservoirs in Yuanba area表2　元坝地区储层岩石类型、沉积微相与测试产能关系Table 2　Relationship between rock type,sedimentary facies and production capacity in Yuanba area

井号测试井段/m主要岩性沉积微相孔隙度/%渗透率/(10-3μm2)测试产量/(m3·d-1)YL7井3461.0～3471.0中-粗粒钙屑砂岩水下分流主河道3.440.033120.80YL12井4364.0～4410.0中-粗粒钙屑砂岩、灰质砾岩水下分流主河道、辫状主河道3.600.06377.17YL18井4520.0～4580.0灰质砾岩辫状主河道3.800.04641.40YL11井4373.0～4397.0灰质砾岩辫状主河道4.200.2410.44YL2井4350.0～4380.0细粒钙屑砂岩水下分流次河道3.100.0143.85YB223井4396.0～4406.0细粒钙屑砂岩水下分流次河道3.100.0193.01YB224井4340.0～4360.0细粒钙屑砂岩水下分流次河道3.100.0172.22YL8井3781.0～3802.5细粒钙屑砂岩河口坝1.900.0021.06

图6　元坝地区钙屑砂砾岩储层碳酸盐胶结物(a)、孔隙度(b)与碳酸盐岩屑含量关系Fig.6　Carbonate cement (a), porosity (b) vs. carbonate debris content in the calcareous coarse clastic reservoirs in Yuanba area

4　结论

1) 元坝地区须三段储层岩石类型主要为钙屑砂岩和灰质砾岩，具有石英、长石含量低，碳酸盐岩屑(碳酸盐砾)含量高的特征。储集空间以粒间溶孔及裂缝为主，物性总体较差，具有低孔、低渗的特征，钙屑砂岩储层物性好于灰质砾岩。

2) 元坝地区须三段储层发育主要受沉积微相、岩石类型、溶蚀作用及裂缝共同控制。水下分流主河道中-粗粒钙屑砂岩为最有利岩相，高含量钙屑组分在后期溶蚀作用及裂缝改造下易形成优质储层。

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(编辑张玉银)

Reservoir characteristics and main controlling factors of calcareous coarse clastic rocks of the third Member of Xujiahe Formation in Yuanba area，northeastern Sichuan Basin

Du Hongquan,Wang Wei,Zhou Xia,Hao Jingyu,Yin Feng

(SINOPECExplorationCompany,Chengdu,Sichuan610041,China)

Several wells have tested moderate-to-high industrial gas flow from the calcareous coarse clastic reservoirs the third Member of Xujiahe Formation in Yuanba Area，Northeastern Sichuan Basin,revealing its great exploration and development potential.However,the reservoir characteristics and main controlling factors are still unclear.Core,thin section and testing data are integrated to study the rock types,reservoir space,physical properties and controlling factors.The reservoirs are dominated by calcareous sandstone and limy conglomerate which featuring in low content of quartz,feldspar and high content of carbonate debris.And the reservoir space is dominated by intergranular dissolved pores,intragranular dissolved pores and fractures.The calcareous sandstone reservoirs are obviously different from the limy conglomerate reservoirs.The former is of fracture-pore type reservoir,while the latter is of pore-fracture type reservoir with low porosity and low permeability.The development of the reservoirs was mainly controlled by sedimentary microfacies,rock types,dissolution and fractures.Besides,for the development of high-quality reservoir,favorable sedimentary microfacies and reservoir rock types are the basis and dissolution and fractures are the key factors.The medium-coarse grained calcareous sandstone in underwater distributary channel is the most favorable faces,as the high content of calcareous component can improve the development of high quality reservoirs through dissolution and fracture modification at late stage.

calcareous coarse clastic reservoir，reservoir characteristics,third Member of Xujiahe Formation,Yuanba area,Sichuan Basin

2015-09-28;

2016-06-22。

杜红权(1985—)，男，工程师，石油天然气勘探。E-mail:dhq4524039@163.com。

国家科技重大专项(2016ZX05002-004)。

0253-9985(2016)04-0565-07

10.11743/ogg20160413

TE122.2

A