Impact of diagenesis on the spatial and temporal distribution of reservoir quality in the Jurassic Arab D and C members, offshore Abu Dhabi oilfield, United Arab Emirates

Abstract

This study is based on petrographic examination (optical, scanning electron microscope, cathodo-luminescence, backscattered electron imaging, and fluorescence) of 1,350 thin sections as well as isotopic compositions of carbonates (172 carbon and oxygen and 118 strontium isotopes), microprobe analyses, and fluid inclusion microthermometry of cored Jurassic Arab D and C members from 16 wells in a field from offshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp with barrier islands and distal slope setting. Petrographic, stable isotopic and fluid-inclusion analyses have unraveled the impact of diagenesis on reservoir quality of Arab D and C within the framework of depositional facies, sequence stratigraphy, and burial history. Diagenetic processes include cementation by grain rim cement and syntaxial calcite overgrowths, formation of moldic porosity by dissolution of allochems, dolomitization and dolomite cementation, cementation by gypsum and anhydrite, and stylolitization. Partial eogenetic calcite and dolomite cementation has prevented porosity loss in grainstones during burial diagenesis. Dolomitization and sulphate cementation of peritidal mud are suggested to have occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones was driven by seepage reflux of lagoon brines formed during major falls in relative sea level. Recrystallization of dolomite occurred by hot saline waters (Th 85–100°C; and salinity 14–18 wt% NaCl). Anhydrite and gypsum cements (Th 95–105°C; fluid salinity 16–20 wt% NaCl), were subjected to extensive dissolution, presumably caused by thermal sulfate reduction followed by a major phase of oil emplacement. The last cement recorded was a second phase of anhydrite and gypsum (Th 95–120°C; 16–22 wt% NaCl), which fills fractures associated with faults.

Multi-scale Quantitative Diagenesis and Impacts on Heterogeneity of Carbonate Reservoir Rocks

“…The second aspect of diagenesis also concerned quantifying techniques, but at a larger, reservoir scale. Here, we benefited from collaboration with the Petroleum Institute in Abu Dhabi (UAE) in order to analyze a huge petrographic and petrophysical database across an oilfield (Morad et al 2012). We constructed maps showing the proportional distribution of diagenetic phases (such as dolomite and anhydrite) and relative abundance of cements, such as syntaxial calcite overgrowth (Nader et al 2013).…”

“…16 Three well-logs from southeast outer ramp to the northeast inner ramp across the investigated field, showing the rock texture distribution of the investigated parasequence set (PS3, red outline)fromMorad et al (2012) . .…”

Multiple Dolomitization Episodes In Deep-Water Limestones of the Triassic Lagonegro Basin (Southern Italy): From Early Reflux To Tectonically Driven Fluid Flow

“…Examples of the first possibility have been discussed in literature (e.g. Roure et al 2005;Morad et al 2012), while other field studies have provided documentation of dolomite textural heterogeneities controlled by former sedimentary features (e.g. Gasparrini et al 2006;Iannace et al 2013).…”

A Multidisciplinary Modeling Approach to Assess Facies-Dolomitization-Porosity Interdependence in a Lower Cretaceous Platform (Northern Spain)

“…The quality evolution of carbonate reservoirs may strongly depend on depositional environments and associated facies, with their intrinsic textures and primary mineralogy (Moore 2001, Ali et al 2010, Morad et al 2012, Nader 2015. These syn-sedimentary features may induce different early diagenetic evolutions, which in turn are commonly reflected in the distribution of burial diagenetic overprints.…”

“…Dolomitization is a common diagenetic process affecting carbonate reservoirs, and numerous studies have focused on the origin of dolomites in different settings (see Warren [2000] and Machel [2004] for a review). Understanding the petrophysical changes induced in the original lithologies by dolomitization is a crucial topic in hydrocarbon exploration as a result of the possible development of secondary porosity associated with the dolomitization process (i.e., Sun 1995, Saller and Handerson 1998, Morad et al 2012.…”