Assessing Sedimentological Heterogeneity through High Resolution Sequence Stratigraphy and Facies Trend Mapping in Undeveloped Upper Jurassic Reservoirs, Abu Dhabi, UAE.

Abstract

Within the onshore area of Abu Dhabi lies a very large super-giant field with confirmed economic hydrocarbon production occurring from an Upper Jurassic formation. This study forms part of a larger body of work that focuses on addressing key uncertainties relating to the sedimentological heterogeneity and the diagenetic overprint within multiple undeveloped gas reservoirs in this formation.

Sedimentological frameworks were established for each studied reservoir interval based on the description of c.2,300ft of core and previous reports of a further 700ft of core acquired from 9 cored wells following the N-S structural alignment of the asymmetric anticline structure of the field. In addition, a dataset of c.250 standard and a small subset of polished thin sections have been used to calibrate core textures, fabrics, sedimentology and petrographic characteristics of the sedimentary rocks. Also, SEM and cold cathodoluminescence analyses have been performed to refine the pore architecture and cement stratigraphy respectively. From the core description and reports, the carbonate fabrics have been classified using Dunham (1962) and Embry and Klovan (1971) carbonate textures along with a set of qualifiers to characterise depositional lithofacies. Individual lithofacies are then grouped into a suite of environmental lithofacies associations representing a range of depositional settings based on genetic similarities and by analogy with modern and known ancient environments.

The Upper Jurassic reservoir has a broad bathymetric range of depositional settings suggesting a relatively high angle carbonate ramp system. The succession records the transgression from an outer ramp setting through a range of inner ramp (oolitic/grain shoal and lagoonal) to intertidal and supratidal sabkha settings. The Upper Jurassic formation is divided into 4 members, with a different character; from the base, the Y unit comprises stacked mud-dominated fabrics interpreted as offshore sediments with variable inner ramp contributions as tempestites. The X unit is the first of three large scale carbonate-evaporite couplets, comprising two smaller-scale couplets, the carbonate being dominantly intertidal with minor subtidal excursions before being capped by nodular supratidal anhydrites. The W unit comprises multiple small-scale carbonate-evaporite couplets with a dominant intertidal and microbial mat character with notable horizons rich in rhizoliths and marginal supratidal microbial anhydrite. The uppermost V unit comprises 4 small-scale couplets but is dominated by anhydrite, the carbonates being thin, but showing changes from grain-rich beach to intertidal and very rare subtidal environments.

The vertical stacking of lithofacies associations provided an insight into the depositional evolution and, along with the recognition of key surfaces enabled the generation of a time based sequence stratigraphic framework for each interval. Calibration of these surfaces with wireline log responses allowed the definition of sequence stratigraphic surfaces (e.g. sequence boundaries, flooding and maximum flooding surfaces where appropriate) to uncored intervals in each reservoir unit. In order to provide quantitative data for assessing lateral facies distribution, lithofacies associations were interpreted in the uncored wells based on a combination of wireline log responses, stratigraphic principles and semi-deterministic depositional models derived from the cored intervals. The production of facies trend maps for each depositional sequence has been based on plotting geologically conditioned boundaries that mark the changing proportions of dominant facies across the field. These maps are supported by reservoir architecture cross sections that show 2D vertical and lateral facies relationships.

The sequence stratigraphic framework and the facies mapping process establish the geological constraints for lithofacies association spatial distribution and thus mitigate uncertainties and establish additional sedimentological controls for properties population in the geological model that forms part of the larger study.