鄂尔多斯盆地中东部地区石炭系本溪组煤岩气储层特征

Reservoir characteristics of Carboniferous Benxi Formation coal-rock gas in the central and eastern Ordos Basin

近年来,鄂尔多斯盆地多口风险探井在石炭系本溪组煤岩中获得工业气流,展示出该盆地煤岩气良好的勘探前景,但关于该盆地煤岩气储层特征的研究较少,煤岩气储层分布规律认识不清,制约了下一步勘探部署。为此,以本溪组8号煤为例,综合岩心、扫描电镜及物性等分析化验资料,对盆地深层煤岩的宏观煤岩类型、显微组分、煤质特征、孔-裂隙发育特征、物性特征和甲烷吸附能力等储层特征开展了系统研究,建立煤岩气储层评价标准,并预测了煤岩气的有利分布区。研究结果表明:①8号煤以原生结构的光亮煤、半亮煤为主,显微组分中镜质组含量高,灰分平均值为17.77%,镜质体最大反射率为1.08%~2.23%,以肥煤-贫煤为主。②8号煤储集类型以胞腔孔、气孔和裂隙为主,其中裂隙包括宏观割理和微裂隙;孔裂隙体积分布以“U”字形结构为主,微孔贡献最大,其次为微裂隙;比表面积分布呈“L”字形,呈现出微孔单峰。③8号煤氦气法孔隙度平均值为6.25%,渗透率平均值为4.21 mD,煤岩在地层状态下具有良好的渗透性。④8号煤空气干燥基兰氏体积为7.79~26.08 m^(3)/t,平均值为18.60 m^(3)/t,兰氏压力为2.03~4.17 MPa,平均值为3.12 MPa,煤岩吸附能力与灰分含量呈负相关性,与成熟度呈正相关性。结论认为,鄂尔多斯盆地本溪组8号煤发育2类有利区,其中横山-米脂-绥德一带为Ⅰ类有利区,面积约11200 km^(2);乌审旗西和榆林北为Ⅱ类有利区,面积约13300 km^(2),2类有利区资源量超过4.5×10^(12) m^(3),可形成煤岩气规模增储上产的现实领域。

In recent years,multiple wildcat wells in the Ordos Basin have obtained industrial gas flows from the coal rocks of Carboniferous Benxi Formation,which reveals the promising prospect of coal-rock gas exploration in this basin.However,the characteristics of the coal-rock gas reservoirs in the Ordos Basin are less researched,and their distribution patterns are not understood clearly,which restricts the next exploration arrangement.Taking No.8 coal of Benxi Formation as an example,this paper systematically investigates the reservoir characteristics such as macroscopic coal rock type,maceral,coal quality characteristics,pore-fracture development characteristics,physical characteristics and methane adsorption capacity of the deep coal rocks in the Ordos Basin,based on the assay data like core,SEM and physical properties.In addition,the coal-rock gas reservoir evaluation criteria are established,and the favorable distribution areas of coal-rock gas are predicted.The following results are obtained.First,No.8 coal is mainly composed of bright and semi-bright coal of primary structure,with a high proportion of vitrinite micro-components,an average ash content of 17.77%,and a maximum vitrinite reflectance of 1.08%-2.23%.Fat coal-lean coal is dominant.Second,the storage space in No.8 coal is mainly acted by cellular pores,stomata and fractures,among which,the fractures include macro cleats and micro fissures.The volume distribution of pores and fractures is characterized by a"U"shaped curve,with micropores contributing most significantly,followed by micro fissures.The distribution of specific surface area presents an"L"shape with a micropore single peak.Third,the average porosity of No.8 coal measured by the helium method is 6.25%,and the average permeability is 4.21 mD,indicating a good permeability of coal rock in the formation state.Fourth,the Langmuir volume of No.8 coal ranges from 7.79 to 26.08 m³/t under the air-dried base,averaging 18.60 m³/t,with Langmuir pressure between 2.03 and 4.17 MPa,averaging 3.12 MPa.The adsorption capacity of coal rock is negatively correlated to ash content,but positively correlated to maturity.In conclusion,there are two types of favorable areas developed in No.8 coal of Benxi Formation in the Ordos Basin.The Hengshan-Mizhi-Suide zone is type I favorable area,covering approximately 11200 km²,and Wushen Banner West and Yulin North are type II favorable areas,covering about 13300 k m².These two types of favorable areas have resources over 4.5×10^(12) m^(3),and can be a realistic field for large-scale increase of coal-rock gas reserves and production.