Reservoir characterization of late Cenomanian Abu Roash “G” member in Sitra field, North Western Desert, Egypt
A R T I C L E I N F O
Received 00 December 00
Received in revised form 00 January 00
Accepted 00 February 00
Abu Roash “G” Member
Flow Zone Indicator
A B S T R A C T
Reservoir characterization of the Abu Roash “G” Member has been studied by integrating petrographical and petrophysical analyses from wells in the Sitra oilfield at the North Western Desert of Egypt. The sandstones of the Abu Roash “G” Member are predominately subarkose arenite to quartz arenite exhibits a wide range of porosities from 1.2 % to 23.8 %, and permeability varies from 0.008 to 126 milli Darcy. Diagenetic features that influenced the reservoir-quality evolution include mainly the mechanical compaction, pressure solution, cementation and the dissolution of framework grains and cements. Compaction and cementation are generally the main factors responsible for the reduction of porosity and permeability in the upper Abu Roash “G” sandstones, whereas the main porosity enhancing include formation of secondary inter- and intra-particle pores by dissolution of detrital feldspars grains. In addition to, minor shrinkage porosity through the dehydration of glaucony pellets is locally recorded. The main porosity preservation in the upper Abu Roash “G” sandstones include the scattered patches of carbonate cement which prevented the compactional collapse of sandstone framework, and grain-coating chlorites that inhibit the precipitation of quartz overgrowths during burial. Statistical analyses including histogram, probability plot, and hierarchical clustering algorithm of petrophysical data based on Hydraulic Unit approach (HFU) were used to evaluate the reservoir characteristics of the Abu Roash “G” Member. The lateral and vertical distribution of the HFUs in the two analyzed wells reflects the high heterogeneity of the reservoir in the Abu Roash “G” sandstones, and could be attributed to the diagenetic processes which enhance or reduce reservoir properties
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Reservoir characterization is a complex process to identify reservoir properties by establishing interdisciplinary relationships from pore to basin scale (Schatzinger and Jordan, 1999). The spatial distribution of rock properties is the key concept in enhancing reservoir characterization (Bhatt et al., 2001; Lopez and Davis, 2010). The important geologic factors that control reservoir quality in oil and gas reservoirs are porosity and permeability. These factors include primary depositional texture and post-depositional processes of burial diagenesis. Diagenesis controls the final geometry of the pore structure, grain orientation and packing, and the degree of cementation and clay filling of pore spaces (Slatt, 2006). Deep reservoir quality in sandstones is the cumulative product of depositional, shallow diagenetic, and deep-burial diagenetic processes (Ajdukiewicz and Lander, 2010; Bjørlykke and Jahren, 2012; Goldstein et al., 2012).
Hydraulic flow unit approach (HFU) was first proposed by Amaefule et al. in 1993 to identify and characterize rock types, based on geological and physical parameters at pore scale. This concept has been widely used in reservoir characterization and permeability prediction studies (Amaefule et al., 1993; Abbaszadeh et al., 1996 ; Soto et al., 2001; Svirsky et al., 2004; Shahvar et al., 2010).
The Sitra field is located in the western part of the Abu Gharadig Basin in the northern Western Desert of Egypt. The exploration history of the area dates back to 1982 following hydrocarbon discovery of upper Cretaceous reservoirs in Sitra 1-1, Sitra 3-1 and Sitra 5-1 exploratory wells. The production started in 1990 from Sitra 3/5 Bahariya reservoir. The present study aims to investigate the reservoir characteristics of the Abu Roash “G” member sandstones by integrating the petrographic characteristics and the types and distribution of diagenetic alterations.
2. Geological setting
The subsurface of the Western Desert of Egypt (Fig.2.1) is characterized by a complicated system of Mesozoic rift basins that cover an area of approximately 200,000 km2 (Hantar, 1990; Sehim, 1993; Bosworth et al., 2008; Dolson et al., 2014; Bosworth et al., 2015). These basins form a series of several discrete E-W to ENE-WSW and NE-SW oriented half-graben basins initiated in the Jurassic and continued to subside through the Cretaceous, such as the Abu Gharadig, Alamein, Matruh, and Shoushan basins (Sultan and Halim, 1988; Emam et al., 1990; Taha, 1992; Moustafa, 2008; Bevan and Moustafa, 2012; Bosworth et al., 2015). The Abu Gharadig basin is an E-W oriented asymmetric graben, represents one of the most important productive basins in the northern part of the Western Desert. It extends for about 300 Km long and 60 Km wide. The sedimentary cover of this basin ranges in age from Late Jurassic to Miocene. Sharib-Shiba high represents the northern border of the basin, Sitra platform is the southern limit, Kattaniya-Abu Roash high to the east and finally Faghur-Siwa basin to the west (EGPC, 1992; Abdelmalek and Zeidan, 1994). Sitra field is a series of right-stepping en echelon faults strike WNW-ESE defining a series of WNW-ESE trending horsts, whereas Sitra 8 horst which represents the current study area is the main productive block of Sitra field.
The stratigraphic section of Sitra field follows the north Western Desert regime (Fig. 2). The Abu Roash “G” Member which is the main target of this study is conformably overlain by the “F” Member of the Abu Roash Formation and is underlain by the Bahariya Formation. It is composed mainly of shale and sandstone with subordinate limestone. The Abu Roash “G” Member is divided into two main units; upper and Middle/lower units separated by transgressive Intra Abu Roash “G” carbonate marker which is laterally very extensive layer in Abu Gharadig basin. This study focus on the upper unit as it represents the main reservoirs of Abu Roash “G” Member in Sitra field.
3. Samples and methods
Samples used in this study were taken from conventional cores from the Abu Roash “G” member, from 2 wells in the Sitra area. The samples are from depths of 3014-3122 m. The Analytical techniques utilized in this study, include thin-section petrography, scanning electron microscope (SEM) and X-ray diffractometry (XRD) have been done at the facilities of EREX Services Company, Egypt. A total of 31 thin sections preparation involved vacuum impregnation with blue dyed resin to facilitate the recognition of porosity and stained with a mixed Alizarin Red-S and Potassium Ferri-Cyanide solutions to identify the carbonate minerals. Sandstone classification were petrographically classified according to the established rock classification schemes of (Pettijohn et al., 1987). For clay mineral identification, the samples were analyzed for the whole rock mineralogy, to obtain a semi-quantitative measurement of the total mineral content of the rock. Although some clay minerals are evident in whole rock diffractograms, the most satisfactory method for their quantification is to extract and separately analyze the clay fractions. Representative samples were viewed under a scanning electron microscope (SEM), to confirm the identification of the different clay minerals, cement morphology, pore geometry and paragenetic relationships. SEM samples were first cleaned in cold Xylene to remove hydrocarbon residues, then mounted on standard aluminum SEM stubs and coated with gold using a sputter coater. Helium Porosity and Gas permeability of the Abu Roash “G” sandstones were carried out at Corex Services Limited Company, Egypt.