AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Applicability of Bentonites to Early Development Areas, TOC Prediction and Quantification of Lateral Facies Variation: A Case in Vaca Muerta Formation (Argentina)


Bentonites (layers of volcanic ash diagenetically transformed in clay) are present in several source rocks, they appear as horizons 1 cm thick in average and may represent 2-6% of the stratigraphy. Bentonites are useful in Exploration to: 1) identify early development areas, 2) predict TOC, and 3) quantify lateral facies variability. Below are observations, based on twenty two cores and an outcrop, and discussion on the application of the bentonites to the case of the Vaca Muerta Fm, Neuquen basin (Tithonian, Argentina): 1) IDENTIFICATION OF EARLY DEVELOPMENT AREAS Within the characteristic clinoforms of Vaca Muerta Fm, bentonites are preferentially recorded in the bottomset where their frequency is the highest (up to a bentonite every 18 cm). Bentonite frequency decreases moving upward along the clinoform (66-91 cm in the foreset) and bentonites disappear in the topset, likely because of increased bottom currents, hampering the deposition of the ashes, and to increased bioturbation, obliterating the potentially-deposited bentonites. The analyses of the bentonite frequency help determine the physiographic sector in a clinoform. 2) TOC PREDICTION The higher the preservation potential of the bentonites, the less the bioturbation; this observation implies that the biological activity obliterates the bentonites. Therefore, bioturbation determines the preservation potential of the bentonites. Interestingly, the preservation potential of the bentonites coincides with the preservation potential of the TOC, which decreases from bottomset to topset. 3) QUANTIFICATION OF LATERAL FACIES VARIATION Because bentonites represent timelines, their correlation, based on sedimentological character, thickness and interbedding thickness, allows quantify the lateral facies continuity of the lithologies. Specifically, the self-similar facies within the bottomset indicate that the sedimentary and diagenetic processes were the same along a stretch of at least 20 km, while within the foreset those processes changed in 5km, hence determining different facies beyond that distance.