四川盆地下侏罗统大安寨段页岩油气地质特征及勘探有利区优选

Geological characteristics and favorable exploration area selection of shale oil and gas of the Lower Jurassic Da'anzhai Member in the Sichuan Basin

四川盆地下侏罗统大安寨段是页岩油气(致密油气)分布的主要层系之一,但由于钻探、测试效果均较差,急需开展有利层段地质特征和勘探有利区优选等研究工作。为此,利用野外露头、岩心、测录井及地球物理等资料研究了大安寨段页岩油气地质特征,分析了页岩油气富集的主控因素,并优选了勘探有利区。研究结果表明:①大安寨段为一套介壳石灰岩与黑色页岩的混积地层,页岩矿物类型以黏土为主,其次为石英、方解石,少量长石、黄铁矿,脆性矿物含量中等;②孔隙类型以无机孔隙为主,含少量有机孔,发育介壳壳缘缝、节理缝等多尺度裂缝,物性随介壳含量的增加而降低;③页岩有机质丰度为中等—好,TOC平均值为1.34%,有机质类型以混合型为主,少量腐泥型和腐殖型,R_(o)平均值为1.11%,热演化程度南低北高,整体处于成熟—高成熟阶段,含油性中等,氯仿沥青“A”含量平均为0.26%;④热解游离烃S1值平均为1.27 mg/g,OSI平均值为84.35 mg/g,页岩油气富集的烃源下限是TOC>1.5%,R_(o)>0.9%,纵向上排烃效率较低,含油指标较高段(OSI>100 mg/g)主要分布于厚层页岩中部;⑤可动油饱和度普遍小于20%,受纹理缝和页理缝影响,石灰岩—页岩互层型、页岩夹石灰岩型样品的油可动性好于石灰岩和页岩。结论认为,大安寨段页岩层系具有较好的页岩油气勘探开发潜力,高生烃品质、多尺度裂缝以及高地层压力是盆地页岩油气富集高产的关键因素,有利油气勘探区主要分布于川中地区三台—仪陇以及南充一带。

The Lower Jurassic Da'anzhai Member in the Sichuan Basin is one of the important distribution series of strata of shale oil and gas(tight oil and gas),but its drilling and test results are worse,so it is in urgent need to research the geological characteristics of favorable intervals and select the favorable exploration areas.Based on field outcrop,core,logging,and geophysical data,this paper studies the geological characteristic of the Da'anzhai Member shale oil and gas,analyzes the control factors of shale oil and gas enrichment,and selects the favorable exploration areas.And the following research results are obtained.First,the Da'anzhai Member is a set of mixed strata of shell limestone and black shale.The shale mineral is mainly composed of clay,followed by quartz and calcite,with a small amount of feldspar and pyrite,and the content of brittle minerals is medium.Second,the pore type is mainly inorganic pore,with a small number of organic pores,and multi-scale fractures such as shell edge fracture and joint fracture are developed.The physical properties decrease with the increase of shell content.Third,the abundance of organic matter in shale is moderate to good with average TOC of 1.34%.The organic matter is mainly of mixed type with a small amount of sapropel type and humic type.R_(o) averages 1.11%,the degree of thermal evolution is low in the south and high in the north,and it is overall in the stage of maturity to high maturity.The oil content is moderate with chloroform asphalt"A"content averaging 0.26%.Fourth,the average S1 value of pyrolysis free hydrocarbons is 1.27 mg/g,and the average OSI is 84.35 mg/g.The lower limit of hydrocarbon source for shale oil and gas enrichment is TOC>1.5%and R_(o)>0.9%.The hydrocarbon expulsion efficiency is lower vertically,and the sections with higher oil content index(OSI>100 mg/g)are mainly distributed in the middle of thick shale.Fifth,the movable oil saturation is generally less than 20%.Due to the influence of texture fractures and shale bedding fractures,the oil mobility in the samples of limestone interbedded with shale and shale with interbedded limestone is better than that in limestone and shale.In conclusion,the Da'anzhai Member shale has great potential in shale oil and gas exploration and development,high hydrocarbon generation quality,multi-scale fractures,and high formation pressure are the key factors for the enrichment and high production of shale oil and gas in the basin,and the favorable exploration areas are mainly distributed in the Santai-Yilong and Nanchong areas of central Sichuan Basin.