Deltaic clinothems − Digital data capture, geometries, and reservoir implications

Subsurface reservoir models are typically limited by a lack of spatially accurate geometric data on bedform architecture and geometry, which are key controls on fluid flow. Outcrop analogues have long been utilized as a source of such data, but the capture of sufficiently precise data has always been a challenge. This thesis demonstrates how advances in data capture and computer technology have made possible the collection of three-dimensional, high-resolution, digital geological data generated by ground based laser scanning (lidar) from steep and otherwise inaccessible outcrops. A complete workflow is documented — from outcrop selection through data collection, processing and building of virtual outcrop models (VOMs) — to geological interpretation, data extraction and the building and testing of geocellular models using an industry-standard, reservoir-modelling software. Examples from analogue systems such as the Roda Sandstone, Pyrenees, Spain, and Cretaceous deltaic systems from central Utah, USA, are used to demonstrate the methodology. A parallel objective of the study has been to utilise the developed techniques to improve understand the geometry, genesis and impact of shallow marine, deltaic clinothems. Clinothems are seaward dipping beds, formed when much of the sand in river-dominated deltas is laid down in distributary mouth bars that coalesce during the downstream accretion of the delta to form a delta front. Systematic collection of data from VOMs has allowed for the compilation of a detailed database on clinothems and associated clinoform geometry. Bed thickness and dip measurements have been collected from two ancient, river-dominated deltaic depositional systems, namely the forced regressive Panther Tongue and a high-stand portion of the Ferron Sandstone that both crop out in central Utah, USA. The extensive bed-thickness database of nearly 3000 measurements from 73 individual clinothems at hundreds of meters to about 1000 m. scale has been used to quantify aspects of delta clinothem-beds. A decay gradient parameter permits the thinning of the clinothems to be described as a single number and thus compared between beds, and measurements are used to calculate the average dip angle. Results demonstrate that clinothems in the Panther Tongue are much longer and more shallowly dipping than clinothems in the Ferron Sandstone. Both systems show similar patterns with regards to groups of gradient-values and groups of dip-values: Common boundaries exist between groups defined by gradient values and groups defined by dip values. A cyclic depositional pattern interpreted to be related to autocyclic processes in deltaic mouth bars is documented, and three and nine stratigraphic cycles or bedsets have been identified in the studied parts of the Ferron Sandstone and the Panther Tongue, respectively. The differences between the two systems are attributed to their sequence stratigraphic positions and hence accommodation. Deterministic geocellular reservoir models have been built from surfaces interpreted in the VOMs. The models have been used to test the influence of dipping siltstones heterogeneities on simulated reservoir performance in a reservoir modelling software. Siltstone drapes on clinothem sandstone beds have been modelled and various populations of holes have been placed in the siltstone beds (0, 30, 60, 90 and 100%). The effects of siltstone permeability (1, 0.1, 0.01 and 0.001 mD) have also been tested. A total of 41 individual models were built and flow simulated. Results quantify how the portion of holes governs the production rate/ recovery factor in the Panther Tongue models. Permeability values are more important in the Ferron models, although they are still influenced by the number of holes. Steeper dipping and closer spaced clinothems of the highstand system tract lower the recovery factor by several tens of percent if the related heterogeneities are all or close to continuous and have low enough permeability