How to Determine Effective Source Rocks: Definition and Distribution

Abstract

There is a consensus that the value of TOC can serve as a standard for evaluating the organic richness (quality) of source rocks. However, the base limits of TOC values for effective source rocks vary in different studies. Someone considered a TOC value of 0.4 wt. % as the base limit for an effective source rock, while others hold that the base limit of the TOC values should be 0.5 wt. %, 2.5 wt. % or 1.0% wt. % . The disagreement of the base limit of the TOC values from different scholars shows that any base limit of TOC values does not have universal applicability. For different research areas, the base limit of TOC values should be obtained according to the actual situation of the study area with a statistical method. The amounts of hydrocarbon-generation of source rocks are composed of free hydrocarbon volatilized out of the rock (S1) and the number of hydrocarbons that can be generated (S2). Therefore, before hydrocarbon expulsion occurs, S1 and TOC should have a good linear relationship. Once the hydrocarbon expulsion occurs, the linear relationship between S1 and TOC will change significantly. The TOC value corresponding to the turning point of the trend line is the base limit of TOC values for high-quality hydrocarbon source rocks. The value of S1/ ω (TOC) represents the amount of hydrocarbon generation corresponding to a unit amount of organic carbon, and the value of S1/ ω (TOC) will first rise, due to the hydrocarbon-generation, and then drop due to the hydrocarbon expulsion. Thus, the value of TOC corresponding to the beginning of the reduction of S1/ ω (TOC)value is the base limit of TOC values for high-quality hydrocarbon source rocks . Based on the method mentioned above, the base limit of TOC values for high-quality hydrocarbon source rocks in Lishui Sag was obtained. In Lishui Sag, the base limit of TOC values for effective source rocks vary in different formations, which may be affected by different organic matter input and maturity of the source rocks. The relatively lower base limit of TOC values in E1y formation (0.7 wt. %) than in E1m (0.9 wt. %) and E1l (1.0 wt. %) formations suggests a better hydrocarbon potential for E1y formation. In this study, the distribution of TOC was predicted based on the base limit of TOC values. The ΔlogR method has been widely used to calculate TOC and has proven to be a reliable method to predict the organic richness of source rocks. And in recent years, the artificial neural network (ANN) model was established to calculate TOC and obtained good results in practice. To predict TOC accurately, both of the results by ANN and ΔlogR methods were determined and compared with the measured TOC. The results show that the prediction results of the ANN method are well-matched with measured data, while some of the prediction results by the Δlog R method are higher than the measured data, indicating the prediction results of ANN are more accurate than the Δlog R method. However, the accuracy of the ANN method is based on the fact that there are enough measured values to train the neural network. In fact, in most cases, some of the wells lack enough measured TOC. Therefore, the planar distribution of TOC in source rocks in Lishui Sag was determined by both the ANN and ΔlogR methods. The TOC of source rocks gradually increased from west to the east in western sub-sag in the study area while source rocks in the eastern sub-sag has lower TOC values.

AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019