Analysis of Configuration Characteristics for Skeleton Sand Body with Tight Sandstone Reservoir: A case study of Triassic Chang 6 members in Heshui area,Ordos Basin,NW China

Key words: 
• Ordos Basin /
• Heshui area /
• Chang 6 oil-bearing formation /
• sand body combination types /
• high-quality sand body

Abstract: Deepwater gravity-flow thick sedimentary sand bodies in the Chang 6 oil-bearing formation are developed in Heshui area,but sand particles are relatively compact,and oil accumulates only in a certain areas with poor continuity. To predict the location of high-quality sand bodies,and considering their different genesis and spatial superposition,the skeleton sand bodies were divided into four types:continuous superposition type;interval superposition type;lateral thinning-out type;and sand/mud interbedded type. This was done based on information gained from field sections,log interpretations,casting thin sections,high-pressure mercury injection testing,physical properties and so on,and taking into account previous research results on lake basin evolution,provenance and other related issues. On this basis,structural characteristics of four compound sand bodies in the Chang 6 oil-bearing formation were quantitatively characterized. Combinations of high-quality sand body types were determined and their distribution is discussed. The results show the following. (1) A sand body of the continuous superposition type is mainly composed of sandy clastic flows of average thickness 8.20 m. Most sand bodies of this type developed in the inner fan of a sublacustrine fan. They have good connectivity and are mainly intercalated. A sand body of the interval superposition type is mainly composed of sandy clastic flows,followed by turbidity flow;the average thickness of single sand body is 4.10 m. Most sand bodies of this type developed in the inner and middle fan of sublacustrine fan. They have barriers and are intercalated;connectivity is determined by the thickness of barrier and intercalation. A sand body of the lateral thinning-out type is mainly composed of sandy turbidity flow,followed by slump rock,and the average thickness of single sand body is 1.10 m. Most sand bodies of this type developed in the middle and outer fan of sublacustrine fan. They are either disconnected(or connected but obstructed),and mainly develop barriers. A sand body of the interbedded sand and mud type is mainly composed of turbidity flow,and have an average thickness of 0.38 m. Most sand bodies of this type are developed in the outer fan of sublacustrine fan. They have no connectivity and mainly develop barriers. (2) From the continuous superposition type to sand/mud interbedded type,the sand bodies become smaller with increased heterogeneity,and their physical properties become poorer,and the oil content lessens. Finally,combination types of high-quality sand body are continuous superposition type and interval superposition type. Planar distribution of high-quality sand bodies occur in the NE of the study area in the NE-SW direction and in plan they flakeshaped or in strips. The SW of the study area is basically undeveloped.