Application of element logging to lithologic identification of key horizons in Sichuan–Chongqing gas provinces

Abstract

With the popularization and application of fast drilling technology in Sichuan–Chongqing gas provinces, the returned cuttings are fine and even powdered, so the traditional cutting lithology identification methods are not applicable any longer. In this paper, qualitative lithology identification and quantitative interpretation based on element logging were conducted on the key oil and gas bearing layers in this area according to the principle of elemental geochemistry. The study results show that: (1) different lithologies can be identified easily because of their different element logging characteristics. For example, basalts have the element characteristics of 0.35 < Fe/Si < 0.55 and Ca/(Na + K) <4.00, while sedimentary rocks have Fe/Si < 0.35 or >0.55. (2) Clastic rocks, carbonate rocks, sulphate and transition rocks in the category of sedimentary rocks can be identified based on the element combination of (Al + Si + Fe) and (Ca + Mg + S). Among them, clastic rocks have (Al + Si + Fe) >31%, carbonate rocks have (Al + Si + Fe) <8% and (Ca + Mg + S) <36%, and sulphate rocks have (Al + Si + Fe) <5% and (Ca + Mg + S) >36%. (3) Then, based on the element combination of (Si + K + Ca)+Si/Al and (Al + Si + Fe + K)+(Ca + S)/Mg, sandstone, mudstone (shale), gypsum, dolomite, limestone and transition rocks can be identified. Finally, a qualitative identification chart and a set of quantitative interpretation software of element logging on key horizons in Sichuan–Chongqing gas provinces were developed to make this method convenient for field application. This method was applied on site in ten wells (such as Well MX207) in Sichuan–Chongqing gas provinces. It is indicated that the coincidence rate of lithology identification is in the range of 88.75–95.22% (averaging 92.42%). Obviously, it can satisfy the requirements of fast lithology identification while drilling of key oil and gas horizons in Sichuan–Chongqing gas provinces.