四川盆地大安区块五峰组—龙马溪组深层页岩地质特征与勘探有利区

Geological characteristics and favorable exploration areas of Wufeng Formation-Longmaxi Formation deep shale in the Da'an Block,Sichuan Basin

近年来,四川盆地渝西地区大安区块上奥陶统五峰组—下志留统龙一1亚段连续获得多口高产工业气井,展示了该区块页岩气良好的勘探开发前景。但由于该区块页岩埋藏深,勘探程度低,深层优质页岩段沉积微相类型、储层特征及其空间展布规律不清,页岩气有利区优选依据不充分,制约了该区页岩气的深化勘探与开发。为此,通过岩心观察、扫描电镜、地球化学与物性测试、含气量测定和核磁共振等实验手段分析,结合钻测井资料,研究了大安区块五峰组—龙一1亚段各小层地质特征及储层展布规律,建立了深层页岩气有利区评价标准,优选了深层页岩气勘探有利区。研究结果表明:①高碳硅质页岩微相和高碳含黏土硅质页岩微相是最优质页岩气储层发育的沉积微相,主要发育于五峰组—龙一13小层。②龙一11—龙一14小层孔隙度平均超过4%,储集性好;五峰组—龙一13小层超大孔和裂缝较发育,所占比例分别为66%和25%。③五峰组—龙一13小层脆性矿物平均含量为55%,可压性好;含气量平均值不低于4.0 m^(3)/t,含气条件较好;龙一11—龙一13小层各层平均总有机碳含量超过3.0%,生烃条件好;地层压力系数普遍大于1.90。结论认为:①五峰组—龙一13小层页岩储层品质最优;②Ⅰ类、Ⅱ类和Ⅲ类页岩气有利区资源量分别为3775.78×10^(8) m^(3)、2970.05×10^(8) m^(3)、599.83×10^(8) m^(3);③该区深层页岩气勘探潜力大,是四川盆地重要的天然气产能接替区,其中临江向斜为下一步页岩气勘探的最有利区。

The Da'an Block in the western Chongqing of the Sichuan Basin has seen many high-yield industrial gas wells targeting the interval from the Upper Ordovician Wufeng Formation to the first submember of the first member of Longmaxi Formation(Longl1)of Lower Silurian in recent years,indicating good exploration and development prospects of deep shale gas in this block.Due to great burial depth and low exploration degree of shale in this block,however,the sedimentary microfacies types,reservoir characteristics and spatial distribution laws of deep high-quality shale sections are not clarified,and there is no sufficient basis for the selection of favorable shale gas areas,which restricts the further exploration and development of shale gas in this area.In this paper,the geological characteristics and reservoir distribution laws of each layer of Wufeng Formation–Longl1 in the Da'an Block are investigated based on experimental analysis including core observations,scanning electron microscopy,geochemical and physical property tests,gas content measurement,and nuclear magnetic resonance,combined with drilling and logging data.Then,an evaluation criterion for the favorable areas of deep shale gas is established,and the favorable areas for deep shale gas exploration are selected.And the following research results are obtained.First,high-carbon siliceous shale microfacies and high-carbon clay bearing siliceous shale microfacies are the best sedimentary microfacies for the growth of shale gas reservoirs,and they are mainly developed in the Wufeng Formation–Longl13.Second,the Longl11–Longl14 interval has an average porosity exceeding 4%,indicating their good reservoir performance.Super-large pores and fractures are relatively developed in the Wufeng Formation–Longl13,accounting for 66%and 25%of the total reservoir space,respectively.Third,the Wufeng Formation–Longl13 has an average brittle mineral content of 55%,indicating their good fracability.The average gas content is greater than or equal to 4.0 m^(3)/t,indicating better gas bearing conditions.The Longl11–Longl13 interval has an average TOC exceeding 3%,indicating good hydrocarbon generation conditions.The formation pressure coefficient is generally greater than 1.90.In conclusion,the shale reservoirs in the Wufeng Formation-Longl13 layer are of the best quality.What's more,the resources in Type I,Type II,and Type III favorable shale gas areas are estimated to be 3775.78×10^(8) m^(3),2970.05×10^(8) m^(3),and 599.83×10^(8) m^(3),respectively.Furthermore,the Da'an Block has great potential in deep shale gas exploration,and it is an important replacement area of natural gas productivity in the Sichuan Basin.Specifically,the Linjiang syncline is the most favorable area of shale gas exploration in the next step.