元素录井在川渝地区关键层位岩性识别中的应用

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

随着川渝地区快速钻井工艺的推广应用,返出岩屑呈细碎甚至粉末状,传统的岩屑岩性识别方法已不能满足要求。为此,根据元素地球化学原理,对该区关键层位开展了元素录井岩性定性识别及定量解释工作。结果表明:(1)不同岩性元素录井特征差异明显,易于识别,如玄武岩为0.35<Fe/Si<0.55且Ca/(Na+K)<4.00,而沉积岩Fe/Si<0.35或Fe/Si>0.55;(2)沉积岩中可依据(Al+Si+Fe)和(Ca+Mg+S)组合识别出碎屑岩、碳酸盐岩和硫酸盐岩及其过渡岩类,其中(Al+Si+Fe)>31%者为碎屑岩,(Al+Si+Fe)<8%且(Ca+Mg+S)<36%为碳酸盐岩,而(Al+Si+Fe)<5%且(Ca+Mg+S)>36%则为硫酸盐岩。进一步可根据(Si+K+Ca)和Si/Al、(Al+Si+Fe+K)和(Ca+S)/Mg识别出砂岩、泥(页)岩、石膏、白云岩、石灰岩及其过渡岩类。为便于该方法在现场应用,建立了该区关键层位元素录井岩性识别定性图版并开发了定量解释软件。在川渝地区MX207井等10口井现场应用的效果表明:该方法岩性识别符合率介于88.75%~95.22%,平均符合率达92.42%,能够满足该区关键油气层位随钻岩性快速识别的要求。

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, qual- itative 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. It is shown that different lithologies can be identified easily be- cause of their different element logging characteristics. For example, basalt has the element characteristics of 0.35 〈 Fe/Si 〈 0.55 and Ca/ （Na＋K） 〈 4.00, while sedimentary rocks has Fe/Si 〈 0.35 or 〉 0.55. Clastic rocks, carbonate rocks, sulphate rocks 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）. It is clastic rocks if （Al＋Si＋Fe） 〉 31%, carbonate rocks if （AI＋Si＋Fe） 〈 8% and （Ca＋Mg＋S） 〈 36%, and sulphate rocks if （Al＋Si＋Fe） 〈 5% and （Ca＋Mg＋S） 〉 36%. Then, based on the element combination of （Si＋K＋Ca）＋Si/A1 and （Al＋Si＋Fe＋K）＋（Ca＋S）/Mg, sandstone, mudstone （shale）, gypsum, dolomite, limestone and transition rock lithology are identified. Finally, a qualitative identification chart and a 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 and 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.