Petrographic and Diagenetic History of Creataceous/Paleozoic Reservoirs and Their Impact on The Permeability Distribution Obaiyed Field, Western Desert, Egypt.
Authors: Wahdan Maamoun , Rahman Hassan Abdel , El Zefzaf Taher 
Published: April 2013

A review of the available data of the Obaiyed field was carried out by Bapetco as part of the 2002 FDP update and the last review of Oct.2004. The Obaiyed field is a complex field, characterized by large uncertainties in permeability distribution and accordingly connected GIIP. The objective was to produce an integrated diagenetic and sedimentological model that can be utilized in 3D reservoir modelling. The study has resulted in a comprehensive 3D model of an estuarine, incised valley depositional system. Geological, Petrographical, sedimentological and Reservoir Engineering data have been integrated to create model realizations reflecting extreme scenarios of the permeability distribution within the field in an attempt to define their connectivity. The majority of the sandstone were deposited as bars within the estuary. The occasional higher permeability layers correspond to petrographic differences in the sandstones that relate to depositional sub-environment. Rock samples have been taken from the Paleozoic/Mesozoic in order to assess the diagenetic history, its impact on reservoir quality, and potential petrographic criteria by using the following analyses: Rock composition, Thin section petrography, SEM-analysis, XRD;Clay Mineralogy and Illite crystalinity;Age dating and Age of diagenesis: K/Ar-dating of Illite/Kaolinite concentrates.

The permeability and saturation distribution are a function of the diagenetic history of the field, causing the Paleozoic sandstones, which underlie the Mesozoic Lower Safa, to have a markedly lower permeability and generally lower saturation. The Obaiyed gas/condensate field presents the challenging combination of a complex subsurface and a short timeframe to fully develop the field and to deliver committed daily quantities. The field was discovered in 1992, subsequently 4 appraisal wells were drilled between 1994 and 1996. After issuing the initial FDP in 1996 a development campaign was started in 1997, first gas was produced in 1999.

After drilling 28 wells in the area subsurface uncertainties have been reduced significantly, the current updated FDP focuses on the development of reserves locked in the tight parts of the field as a means to maximize profitability of the field. Currently over 50% of the in-place gas is locked in tight gas sandstones in the Obaiyed field.

In order to rule out other causes of poor productivity, the formation damage is the mechanism responsible for poor productivity. As this can only be done by ensuring no damage during drilling

Over the next two years, drilling activity in Bapetco will be increased, mainly because of the upcoming drilling campaign to develop the NEAG field In Abu El Gharadig Area and Obaiyed Area. Initial development plan will comprise drilling of 8-10 development wells in both Areas.

The majority of the sandstone was deposited as bars within the estuary. The occasional higher permeability layers correspond to petrographic differences in the sandstones that relate to depositional sub-environment. Rock samples have been taken from the Paleozoic/Mesozoic in order to assess the diagenetic history, its impact on reservoir quality, and potential petrographic criteria by using the following analyses: Rock composition, Thin section petrography, SEM-analysis, XRD;Clay Mineralogy and Illite crystalinity;Age dating and Age of diagenesis: K/Ar-dating of Illite/Kaolinite concentrates.

The permeability and saturation distribution are a function of the diagenetic history of the field, causing the Paleozoic sandstones, which underlie the Mesozoic Lower Safa, to have a markedly lower permeability and generally lower saturation. A comprehensive data gathering and interpretation campaign has been progressed, including more cores, image logs, nuclear magnetic resonance tools, age dating, palynological and Paleomagnetic analysis to improve understanding of the reservoir permeability distribution. The Illite distribution is the most important control on the permeability values (quantitative and/or qualitative). Over the next two years, drilling activity in Bapetco will be increased, mainly because of the upcoming drilling campaign to develop and sustain the deliverability of the Obaiyed field. Initial Obaiyed development comprised drilling of 18 development wells between Nov.1997 and end of 2000, and the upcoming campaign (6-8 wells) between 2002 and 2004. The main objectives of these wells are to ensure availability of at least 40 MMscf/d (after fraccing) per each well to allow the DCQ of 300 MMscf/d to be safely delivered.

During the past two years Bapetco has carried out field reviews and updating the development plans for Obaiyed fields, with the aim to re-assess the optimum life cycle development of these resources. As part of these reviews, 3-D reservoir modelling and full field simulation studies were carried out using tools such as Geocap, Logic, MoReS and Pegasus. The studies have resulted in comprehensive 3-D models of the coastal plain and shallow marine reservoir sequences in these fields. Geological, petrographical, petrophysical and reservoir engineering data have been integrated to create model realisations reflecting uncertainties in a range of parameters such as reservoir architecture, petrographic and diagenetic process, structure and fluid contents in an attempt to define the range in the hydrocarbons initially in place (HCIIP) and static connectivity. 3-D permeability and porosity models have been generated for all reservoirs and were subsequently used in MoReS reservoir simulation studies. These models are being used actively for development planning purposes.

To date, some 18 well proposals have been matured on the basis of these studies, of which 17 have been drilled. This paper summarizes the results of these wells and presents the main learning points gathered over the past years. Although most wells were successful, there were also disappointments. Following the update of the models with the results of these wells, scope for horizontal well is opened. At the same time however, additional gas volumes were identified to the north and possibly further appraisal well have consequently been raised. 3D static and dynamic reservoir modelling have now become part of Bapetco's day to day activities. Generally, use of these tools has facilitated the preparation of proposals and has shortened the approval cycle due to the increased technical standards of the proposals. This has allowed us to successfully progress the intensive activity program of the last few years. Over the next two years, drilling activity in Bapetco will be increased during the drilling campaign to develop the Obaiyed field, Egypt's most prestigious gas development. Initial Obaiyed development comprised drilling of 18 development wells between 1997 and 2000 and construction of surface facilities. Obaiyed is a complex field, characterized by large uncertainties in GIIP and permeability distribution. Rapid interactive feedback between actual and predicted well results is being applied to allow proper management of the next development campaign.