Khalid Al-Ramadan

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

High-resolution facies and porosity models of the upper Jurassic Arab-D carbonate reservoir using an outcrop analogue, central Saudi Arabia

Subsurface models of hydrocarbon reservoirs are coarse and of low resolution when compared with the actual geologic characteristics. Therefore, the understanding of the three-dimensional architecture of reservoir units is often incomplete. Outcrop analogues are commonly used to understand the spatial continuity of reservoir units. In this study, a Late Jurassic outcrop analogue for the Arab-D reservoir of central Saudi Arabia was used to build a high-resolution model that captures fine geologic details. Subsurface reservoir lithofacies were matched with those from the studied outcrop, and porosity values derived from published core and well log data from the Ain Dar, Uthmanyah, and Shudgum areas of the Ghawar Field, eastern Saudi Arabia, were then applied to the equivalent lithofacies in the outcrop. Maximum, minimum, and average subsurface porosity for each lithofacies were distributed in the facies model using a geostatistical algorithm to produce nine porosity models for the field data. Several realisations were run to visualise the variability in each model and to quantitatively measure the uncertainty associated with the models. The results indicated that potential reservoir zones were associated with grainstone, packstone, and some wackestone layers. Semivariogram analysis of the lithofacies showed good continuity in the N-S direction and less continuity in the E-W direction. The high-resolution lithofacies models detected permeability barriers and isolated low porosity bodies within the potential reservoir zones. This model revealed the porosity distribution in areas smaller than one cell in the subsurface model and highlighted the uncertainty associated with several aspects of the model.

Porosity evolution within high-resolution sequence stratigraphy and diagenesis framework: outcrop analog of the upper Jurassic Arab-D reservoir, Central Saudi Arabia

The upper Jurassic Arab-D reservoir is considered as the most prolific reservoir in the Ghawar field in Saudi Arabia. Exposed strata equivalent to the Arab-D reservoir was investigated and evaluated to establish a relationship between lithofacies, sequence stratigraphy, diagenesis, and porosity evolution within the Arab-D reservoir analog. The study revealed eight lithofacies which interpreted to have been deposited in an open-marine lower-slope and upper-slope of a ramp platform, ramp-crest, distal to proximal lagoon and tidal flat environments. The diagenetic and paragenetic analysis showed early marine dissolution and cementation, followed by replacement of aragonite and high Mg calcite to low Mg calcite. Porosity enhancement started with shoaling-up of the system. This produces most of the moldic and intraporosity in the study area. Although the outcrop section showed a degree of burial compaction, there is no pronounced effect of compaction on porosity reduction. With extensive progradation of the proximal lagoon and inner ramp along with meteoric realm, near-surface dolomitization was formed and resulted in porosity increase. This was very clear at the top of each of the high-frequency sequences of the lower part of the outcrop. Later, the whole system had undergone fracturing which enhanced porosity dramatically. Although, the outcrop has little or no macro porosity due to sub aerial exposure and recent meteoric cementation, the paragenesis study provides a predictive porosity distribution model within a high-resolution sequence stratigraphy framework and its associated diagenetic events. This model could provide better understanding of porosity evolution and valuable guide for subsurface exploration.

Microfacies analysis of Wadi Waqb member (Miocene) in Wadi Aynunah, northwest of Saudi Arabia

Easier access to most of spectacular sedimentological features of Midyan region along the northern coast of Red Sea has enabled a more detailed examination of the Miocene succession that could not have been possible except from deep subsurface drilling. The Wadi Waqb member of Jabal Kibrit Formation hosted the Wadi Waqb reservoir in this region. In this study the microfacies of the exposed discontinuous fringing rhodolith and coral reef complex of Wadi Waqb member that seated unconformably on steep cliffs of granitic basement are investigated. Four microfacies were recognized as follows: MF-1) Quartz-peloid packstone and Red algal bioclastic boundstone, MF-2) Bioturbated rhodophyte-bioclastic grainstone, MF-3) Calc-allochemic sandstone, and MF-4) Intraclastic wackestone.Microfacies 1 and 2 are considered to be deposited at the upper and mid reef front. The depositional environment of the MF-3 is considered to have occurred in periodical flash floods and microfacies 4 is considered to be a lower reef front deposit. Careful microfacies analysis of mixed reefal carbonate and siliciclastic deposits provides a better understanding of how and why carbonate microfacies develop.

Integrated diagenesis and sequence stratigraphic study of tidal sandstones: the Adedia Formation (Cambro-Ordovician), Sinai, Egypt

This work examines the different effects meteoric versus marine diagenesis had on Cambro-Ordovician tidal sandstones during episodes of fluctuating sea level. The distribution of diagenetic fabrics was compared to a sequence stratigraphic framework. Initially, a rise in relative sea level (RSL) resulted in deposition of transgressive systems tract sands directly onto crystalline basement. These sandstones display evidence of limited cementation by marine, grain-fringing dogtooth-like and fibrous calcite. A fall in RSL resulted in the progradation of a tidal flat complex and deposition of highstand systems tract (HST) and lowstand systems tract (braided fluvial) sandstones. Contemporaneous meteoric-water flux into sands of all the systems tracts occurred. Sequence boundaries (SB) are marked by fluvial incision of tidal sands and by the development of palaeosols. Meteoric incursion during sea-level lowstands resulted in the dissolution and kaolinitization of feldspars, micas and mud intraclasts in all systems tracts, but is most extensive in HST sandstones below the SB. The effect of meteoric-water flux on the dissolution of marine calcite cements is poorly known. Mesogenetic alterations include intergranular pressure dissolution and formation of variable amounts of syntaxial quartz overgrowths in all systems tracts. Telogenetic alteration (i.e. weathering) in the sandstones includes the formation of goethite and calcite. Thus, the integration of diagenesis with sequence stratigraphy provides a useful tool with which to understand reservoir-quality distribution in sand-dominated, tidal sediments.