自然伽马能谱测井识别低阻油层的一个实例

Low resistivity reservoir detection using GR spectrometry logs:a case study

珠江口盆地惠—陆低凸起带上LF13-2油田LF13-2-1井古近系恩平组及珠海组等深层几套砂岩储层的常规测井解释为油水同层,但井壁取心为富含油层,由于该油田钻井稀少且缺乏岩心化验、测试等资料,究竟是油水同层还是低阻油层的问题影响了该油田深层油藏类型的准确甄别与评价。本文对该区和邻区古近系砂岩储层的样品岩矿鉴定与测试特征、成岩地质背景重新进行了梳理与分析,并利用自然伽马能谱测井数据中钍、钾等放射性元素的交会图与直方图分析方法,着重对恩平组及珠海组储层中黏土矿物类型、质量分数相对大小和强吸附水型黏土矿物形成连网导电水膜的降阻机理等进行了较深入的探讨,获得对LF13-2-1井的重新解释结论是:2879.5～2885.0m储层段为高束缚水型低阻油层(原测井解释为油水同层),同时也间接验证了邻区惠州凹陷东段取心井的电镜扫描和黏土X衍射结果。

The Oilfield LF13-2 is located in the Hui-lu lower uplift of Pearl River Mouth Basin.In this oilfield,the Enping and Zhuhai formations of Pleogene at the well LF13-2-1 was interpreted as oil-water bearing layers using the conventional well logging data.However,the sidewall core data indicate that these formations are oil-bearing reservoir.It is challenging to determine whether these formations are oil-water bearing or oil bearing with low resistivity due to the sparsity of the wells and the lack of the necessary test on the core data,which makes it difficult to accurately discriminate the reservoir type and evaluate the reservoir in the oilfield.In this paper,the test on the core samples from the zones of interest in this oilfield and adjacent one are sorted and analyzed as well as the diagenetic geology.Afterwards,using the histogram and scatter plot of the thorium and kalium from the gamma ray spectrometry logs,the clay mineral type with high adsorbing water,mass fraction and the low-resistivity mechanism caused by the conductive water-film net of the clay mineral are studied.Based on our study,the following new conclusion has been drawn on the formations at well LF13-2-1:the zone in depth from 2879.5 meters to 2885 meters that was considered as oil-water formations is actually the oil layers with rich bound water and low resistivity.This conclusion has been indirectly verified by the scanning electron microscope and X-ray data of the core samples from the east of Huizhou depression in the adjacent oilfield.