鄂尔多斯盆地陇东地区延长组砂岩成岩作用及孔隙演化

Diagenesis and porosity evolution of sandstones in Longdong Area,Ordos Basin

大量岩石薄片、孔隙铸体薄片、扫描电镜与X衍射以及常规物性等分析资料研究表明,鄂尔多斯盆地陇东地区三叠系延长组砂岩主要为成分成熟度及结构成熟度较低的长石质岩屑砂岩、岩屑砂岩和长石砂岩。优质储层主要发育于粗粒富含刚性颗粒的砂岩中,这类砂岩在埋藏过程中由于早期快速埋藏加之泥岩、板岩及千枚岩岩屑含量高,经历了强烈的压实作用,使其由原始孔隙度大约为39%降低至18.5%;此后发生石英颗粒次生加大及绿泥石胶结,使孔隙有一定程度的损失(大约3.5%);紧随其后的是方解石及白云石碳酸盐胶结,损失孔隙度5%;后来出现碳酸盐胶结物与长石及岩屑的溶蚀作用,新增加孔隙度4%～8%,最晚期的是含铁方解石和含铁白云石胶结充注溶蚀孔隙和部分原生孔隙。

Researches based on the pore casting thin sections, scanning electron microscopy （SEM） and X-ray diffraction, combined with the analysis of physical properties, have shown that the types of the sandstone reservoirs of Triassic Yanchang Formation in Longdong Area, Ordos Basin, are lithic arkose sandstone, feldspathic lithic sandstone and arkose with low compositional and low structural maturities. The high quality reservoirs were mainly developed in some coarse grained sandstones rich in rigid grains. The primitive porosity of this sandstone is approximately 39% from some empirical formula. During burring, its porosity decreased to 18. 5% due to the fast burial and richness in the shale, slate and phyllite debris. Afterwards, the porosity continually decreased in some extent （3.5%） as a result of the overgrowth of quartz and chlorite cementation. Constantly, the calcite and dolomite cementation contributed a porosity loss of about 5%. Afterwards, the dissolutions of carbonate cement, feldspar and debris made a contribution to the increase of porosity by about 4%-8%. Finally, the dissolution pores and some primary pores were infilled by ferrocalcite and ankerite cement.