Well log response and evaluation of abundance of organic matter in shaly source rocks in Kangjiqi area, Ordos Basin

Abstract:

Based on comprehensive analysis of the well log responses of different type of source rocks from 70 wells, shaly source rocks in upper palaeozoic Taiyuan Formation and Shan1 and Shan2 units of Shanxi Formation were identified and the thickness of source rocks and TOC were estimated using the △logR technique in the northern Ordos basin.

The thickness of shaly source rocks in Taiyuan Formation presents the characteristics of thinning from south to north, but Jin-35 Well in the north shows the thickest shaly source rock with thickness of 31.5m. The second areas with thick source rock are located in Jin 72, 56, 61 well block with thicknesses of 26m, 19m, and 20m respectively. The distribution of TOC (total organic content) in shaly source rock shows the pattern of higher value in the southwest and middle areas (Jin-69and Jin-90 well block) and lower value in the north east area. TOC in Jin-69 well is 8.96% and 8.07% in Jin-62 well.

The thickness distribution of shaly source rocks in Shan1 unit of Shanxi Formation displays the characteristics of thinning from south to north, but on the maximum thickness distribution, Jin-60 Well in the west shows the thickest shaly source rock with thickness of 30m. The distribution of TOC (total organic content) shows a pattern of higher value in the south and middle areas and lower value in the western and northern areas. TOC of Shan1 unit in Jin-68 well is 7.15% and 5.83% in Jin-69 well.

The shaly source rocks in Shan2 unit as a whole is characteristically thicker in southwest area and thinner in the northeast area and thickest at Jin-60 well in the southwest with thickness of 32m. The areas with thicker source rock are located in the Jin 31-58-57 well block and Jin 69 - 32 well block with thickness of 22.5m in Jin-31 well and 22.7m in Jin-32 well. The TOC in Shan2 unit is relatively lower. High TOC value is only present in Jin-94 well with TOC of 5.31% and 6.08% in Jin-48 well, but the distribution area is limited.

Overall, the shaly source rocks in upper Paleozoic Taniyuan Formation and Shan1 and Shan2 units of Shanxi Formation can be used as effective source rocks in Kangjinqi area, providing material for gas accumulation. TOC in upper Paleozoic source rocks shows the trend of gradually becoming poor from Taiyuan Formation, Shan1 unit to Shan2 unit.