四川盆地下寒武统筇竹寺组页岩孔隙特征及物性变差机制探讨

Microscopic pore structure and reasons making reservoir property weaker of Lower Cambrian Qiongzhusi shale,Sichuan Basin,China

四川盆地下寒武统筇竹寺组富有机质页岩分布范围广、厚度大,具备形成页岩气的地质远景,但至今尚未建成工业产能。以多口取心井岩心为基础,利用扫描电镜、核磁共振、比表面积测量和氮气吸附等分析方法,对筇竹寺组页岩储层的孔隙类型、孔隙结构进行研究。结果表明,筇竹寺组储集空间类型单一,以黏土矿物晶间孔为主,有机质孔隙不发育,脆性矿物内孔隙和裂缝孔隙少。基质孔隙主体为孔喉直径低于50nm的中孔和微孔,孔隙度一般为1.4%～3.1%。筇竹寺组物性变差的主要形成机制为：1热成熟度高,有机质碳化加重,有机质孔隙不发育;2成岩作用强,以结晶度高的绿泥石和伊利石等黏土矿物为主,晶间孔减少;3液态烃裂解近衰竭,孔隙内压力降低,有机质孔隙出现塌陷,并在围岩挤压和黏土矿物结晶收缩作用下经压实消失。

The Lower Cambrian Qiongzhusi Formation has characteristics of widespread distribution in the Sichuan Basin and large thickness.It has shale gas geological prospect, but has not yet built industrial ca- pacity.In this paper, scanning electron microscopy analysis, nuclear magnetic resonance, surface area meas urement and isothermal adsorption experiments were used to analyze pore types and pore structures of Qiongzhusi cores.The following results were obtained.The type of reservoir space of the Qiongzhusi For- mation is sole, which mainly contains pores among clay mineral crystals, with few organic pores, whereas very few pores within brittle minerals and between cracks.Matrix pores are mainly composed of mesopores and micropores with diameter less than 50 nm.The porosity is around 1.4%to 3.1 %.The main reasons for bad reservoir property are as follows： （1）high maturity caused carbonization of organic matters and unde- velopment of organic pores ; （2） clay minerals are mainly chlorite and illite, strong diagenesis causes high crystallization of clay minerals and reduction of pores among crystals; （3）gas generation exhaustion of liq- uid hydrocarbon and decreasing pressure in the organic pore lead to the collapse and disappearance in the surrounding rock squeeze and crystallization shrinkage of clay minerals.