The evolution and development of faults in extensional basins controls the geometry and development of carbonate platforms. In such settings, carbonate platforms bounded by faults show facies patterns and stratigraphic geometries which vary along strike with footwall uplift and hanging wall subsidence as a result of fault growth process, as exemplified in the Red Sea syn-rift. Most Saudi Arabian studies and stratigraphic descriptions of syn-rift Miocene carbonates are largely based on examples in NW Saudi Arabia, which limits our understanding of their distribution, controls, diagenesis, and heterogeneity in other areas of Saudi Arabia, including of the Wadi Waqb Member (Jabal Kibrit Formation), which is a proven carbonate reservoir. Carbonates exposed between Al Wajh and Duba on the Red Sea margin are described here for the first time, allowing a more detailed characterization of the different forcing mechanisms involved in the evolution of such carbonate platforms in syn-rift settings.
This study integrates basin scale geology with insights from fieldwork (100s of m to m), sample-scale (cm), and microscopic (nm - mm) evaluation of the facies distributions and, for the first time, the diagenesis and detailed quantitative petrophysical measurements of the physical characteristics of these carbonates. Twelve diverse facies types have been differentiated, described, and interpreted through petrographic analysis. They are organized into four facies associations resting on Proterozoic basement. The platforms are characterised as attached, shallow marine, fault-bounded rimmed carbonate platforms developed across a series of structural relay zones. The study has confirmed, for the first time, the presence of the shallow-water carbonates of the Wadi Waqb Member on another fault block south of the Midyan region, based on Borelis melo melo and fossil assemblages. A first order structural control on their distribution is evident, with platform-top facies assemblages passing rapidly into reworked slope deposits across the terrace-bounding fault.
The study also characterizes a previously undescribed tilt block-bounding fault (with a 3m wide fault core) separating the Wadi Azlam terrace from what herein is named for the first time the Dukhan Basin. Based on the relationship between fault width and displacement, the Dukhan fault block is assumed to have a displacement of km-scale, typically resembling the major faults in the Midyan region. The evolution and geometry of this linked fault zone controlled carbonate geometries within relay zones and on the downthrown hanging wall block. The vertical facies variation reveals a complex interplay between tectonic uplift and global mean sea-level changes during the upper Early Miocene. Sea-level fluctuations are reflected in carbonate geometries and sediment redeposition (periods of submergence and erosion) and demonstrate aggradational-progradational staking patterns similar to those in the Midyan Basin.
Porosity distribution across the fault damage zone indicates that fluid flow was controlled by the structural fabric. In addition, results show that early carbonate dissolution within the fault zone had created a significant connected network of high percentage moldic, vuggy and fracture porosity that was exploited by the subsequent dolomitizing and mineralizing fluids. The petrographical and geochemical observation has shown that multiple phases of dolomitization (D1 and D2) have formed from different fluid sources and are different in each studied section. The dolomitization within the studied succession of the Wadi Azlam and Dukhan sections show coarser, zoned dolomite and Zn-bearing polymetallic minerals which are localised mainly to fault zones.
The study provides the first details of different petrophysical measurements in vuggy carbonates from core plug samples among the Wadi Waqb carbonates. The results help compare between various porosity estimations besides usage of two different gases. Using two different porosimetry gases helps to compare the results from cores with large surface vugs, while transient porosimetry has been used to track the entry of gases into the vugs within the core samples. In addition, steady state permeametry and water saturation measurements are studied. The results revealed the porosity average from all methods ranged from <5% to 31%, with an average of 21.06%, but permeability was notably low due to pore-filling diagenetic cements such as calcite and iron oxides. The platform interior facies exhibited lower porosity and permeability than higher values at the platform margins and in slope deposits, where fractures and vugs enhanced fluid flow. The study findings highlight how the use of different methods is essential to address measurement uncertainties in vuggy and fractured materials with the heterogeneity of Wadi Waqb carbonates.
Results indicate that the geometry of the carbonates together with the geometry of the fracture network, and in particular the distribution of open fractures through time, may have controlled the distribution of fluid-flow conduits for dolomitizing and mineralizing fluids. This study highlights the interplay of various factors affecting the fault-bounded, rimmed platform carbonates and their porosity evolution.
The present study provides novel findings that describe for the first time the stratigraphy of syn-rift Miocene carbonates on the western Saudi Arabian coast. This without a doubt contributes new lithostratigraphic understanding for the region. This research suggests that the Wadi Waqb carbonates at outcrop show high reservoir quality and may be promising as an analogue to subsurface equivalents in Saudi Arabia. The current study will also open windows on other parts of the Saudi Red Sea margin, when using similar techniques and methodologies or combinations of both.
