The Correct Shale-Volume Characterization Increases Hydrocarbon Reserves: Case Study of Cretaceous Formation, Lake Maracaibo, Venezuela

Abstract

Abstract Today, natural fracture and/or connected vuggy systems in carbonate reservoirs contribute significantly to hydrocarbon production. Combining concepts from normal distribution with normalization and soft computing techniques improves quantification of actual shale volumes for a reservoir with complex stratigraphy and natural fractures. This is especially important for cases when very few log curves are available to solve a high number of unknown lithologic variables. We applied our new methodology successfully in the Cretaceous formation, Lake Maracaibo, which is composed primarily of limestones with some dolomites and siliciclastics (glauconitic sandstones, siltstones, and shales). It is common to use the standard gamma ray log (SGR) or total contribution from all three elements—uranium (U), potassium (K), thorium (Th)—as an indicator of shale content. The presence of highly radioactive black organic material and/or natural fractures in the formation results in a big difference from the X-ray diffraction data. This causes an overestimation of shale volume and therefore affects the original oil in place (OOIP) and reserves. We present a novel methodology that combines normal distribution and normalization to predict CGR from SGR and deep resistivity, Rt. We applied the cross correlation technique to validate our methodology, and the model CGR matches the actual CGR very well. Next, we used the elemental capture spectroscopy (ECS) logs to quantify the actual clay volume (Vsh). Then, we used soft computing techniques to develop a shale volume model using CGR and Rt as independent variables and the Vsh from ECS as the dependent variable. Running the model for validation in three wells with ECS achieved a correlation coefficient of 0.9. The average shale volume using our model is 12.5%, much lower than the former linear shale-volume model, which averaged 28.4%. This has had a great impact on the OOIP and reserves of our reservoir, as it would for other complex carbonate reservoirs.