上扬子地区寒武系牛蹄塘组页岩有机质孔隙发育特征及主控因素

Organic matter-hosted pores in the Cambrian Niutitang shales of the Upper Yangtze region:Pore development characteristics and main controlling factors

上扬子地区下寒武统牛蹄塘组页岩气勘探开发程度较低,且不同沉积区域有机质孔隙发育具有明显的差异性。本文针对位于上扬子板块东部鄂西裂陷槽的EYY1井、板块西部浅水陆棚相的W001-4井以及板块北部汉南古陆的SNY1井3个不同沉积区域的钻井样品进行分析,根据矿物组成划分了岩性;利用扫描电镜观察分析了有机质类型及有机质孔隙发育特征,并在此基础上讨论了有机质孔隙发育的主控因素。研究发现:(1)不同区域牛蹄塘组页岩矿物及岩性组成有差异,EYY1井、W001-4井页岩取样段岩性包括硅质页岩、钙质页岩和过渡型页岩3类,而SNY1井取样段岩性均为硅质页岩;(2)牛蹄塘组有机质分为原生有机质和运移有机质,原生有机质均具有较高的C/O比值,平均为6.74;运移有机质C/O值相对较低,平均为2.71;(3)EYY1井与SNY1井有机质孔隙发育较好,面孔率达到6%~28%,而W001-4井有机孔发育较差,面孔率为3%~10%。牛蹄塘组有机孔孔径分布均为单峰型,EYY1井和SNY1井有机质孔隙直径主要分布在2~50 nm,W001-4井样品有机质孔隙大小普遍在5~25 nm;硅质页岩有机质面孔率普遍高于钙质页岩;(4)牛蹄塘组页岩中运移有机质孔隙发育程度好于原生有机质,有机质生烃增孔作用和压实减孔作用相互叠加影响有机孔的发育。黏土矿物是制约页岩有机质孔隙微观发育机制的重要因素,富黏土的硅质页岩可能是寒武系页岩气甜点选择的重要线索。

Shale gas exploration and development is relatively low level in the Lower Cambrian Niutitang Formation in the Upper Yangtze where organic matter-hosted pores(OMPs)developed unevenly across the region.In this paper,shale samples from three wells with different depositional settings—namely well EYY1 in the western Hubei rift trough of eastern Upper Yangtze,well W001-4 in the shallow-water shelf of northern Yangtze,and well SNY1 in the Hannan ancient land of northern Yangtze—were collected and analyzed,and shales were classified according to mineral compositions.Organic matter types and OMP development characteristics were investigated by scanning electron microscope method,and the main controlling factors for OMP development were discussed.The results show that(1)mineral compositions and types of the Niutitang shales vary between different regions.Shale samples from EYY1 and W001-4 were classified into siliceous,calcareous and mixed shales;while in SNY1 only siliceous shale was found.(2)Organic matters in the Niutitang shales were divided into depositional and migrated organic matters.In the three wells,the carbon-to-oxygen elemental ratio(C∶O)in the depositional organic matter(averaging 6.74)was higher than that in the migrated organic matter(averaging 2.71).(3)OMPs were well developed in EYY1 and SNY1,with relatively high surface porosities of 6%28%;while in W001-4 they were poorly developed,with surface porosities ranging between 3%10%.The OMP diameter in the three wells showed a unimodal distribution,centering between 250 nm in EYY1 and SNY1 and generally between 525 nm in W001-4.Organic matter in siliceous shales had the highest surface porosity,higher than in calcareous shales.(4)Migrated organic matter showed higher level pore development compared to depositional organic matter.OMP development was affected by counteraction between pore production from hydrocarbon generation from organic matter and pore reduction from compaction.Clay mineral is another important factor hindering micro-scale OMP development in shales,and siliceous shales rich in clay minerals may be an important clue for the determination of exploration“sweet spots”for Cambrian shale gas resource.