Microfacies and Reservoir Heterogeneity and Architecture of the D5 and D6 Members of Middle Jurassic Carbonates Dhruma Formation in Outcrops, Central Saudi Arabia

Abstract

The well-exposed Jurassic carbonate outcrops in central Saudi Arabia provide an excellent opportunity to characterize the detailed microfacies heterogeneity and stratigraphic architecture within the real inter-well spacing. The current study integrates microfacies and stratigraphic analysis of the D5 and D6 Members in outcrops to define lithofacies, cyclicity, and high-frequency sequences. More than 130 samples were examined from the petrographyical point of view along with high-resolution photomosaic analysis. The studied sections are dipping and having a total thickness of about 58 meters. The outcrop photomosaic was used to map the reservoir bodies architecture and continuity. Nine lithofacies types were identified namely: (1) skeletal peletal spiculitic wackestone, (2) peloidal echinoderm packstone, (3) fissile shale, (4) peloidal spiculitic echinoderm pack-grainstone, (5) cross-bedded peloidal skeletal oolitic grainstone, (6) oolitic grainstone, (7) intraformational rudstone, (8) skeletal peloidal foraminiferal packstone and (9) skeletal foraminiferal wackestone. Fissile shale represents the most dominant lithofacies type in studied intervals. These lithofacies types were grouped into five major carbonate paleoenvironments that range from distal-to-proximal ramp setting. The high-resolution outcrop photomosaic analysis shows the lateral continuity of strata which extends to about 60 m vertically, and laterally for a distance of more than 680 m without significant lateral facies change. The vertical stratigraphic analysis revealed 53 cycle and cycle sets with 5^th to 6^th order. The parasequences range in thickness from centimeters to six meters with an average of 1.5 m. The latter were stacked to form four high-frequency sequences with thickness range from 1 up to 14 meters, and generally showed an increasing thickness and overall shallowing upward trend. The Jurassic maximum flooding surface (MFS J30) was placed within a thick transgressive shale/mudstone unit. The entire succession of D5 and D6 Members represent a single depositional sequence of 3^rd order magnitude. The findings of this high-resolution outcrop analog scale analysis provide a detailed insight into reservoir description, architecture and quality prediction within the actual inter-well spacing of the D5 and D6 Members in the subsurface. Consequently, this might also serve to optimize reservoir exploration and development of subsurface equivalent reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018