Silva Gonzalez, Patricio (Stratum Reservoir) | Fernandes, Melissa (Stratum Reservoir) | Siddiqi, Saad (Stratum Reservoir) | Hannon, Loay (Stratum Reservoir) | Steiner, Stefan (ADNOC Upstream) | Chitrao, Amogh (ADNOC Upstream)
Rock typing represents an important tool for classifying clastic reservoir rocks, especially in reservoirs showing low permeability and low porosity values. For this study we first approach these challenges from a geological perspective and subsequently translate the results of traditional facies and RQ analysis into rock types by integrating previously obtained results with mercury injection capillary pressure (MICP) data. The rock typing workflow proposed by Rushing et al. (2008) is applied to the low porosity-low permeability Unayzah Formation penetrated in three offshore wells, UAE. The characterisation of this type of low reservoir quality (RQ) rock confronts the geologist with some challenges. The Unayzah Formation is an important clastic reservoir unit in the Middle East, especially in the UAE. It is characterized by low porosity and low permeability values and therefore, according to Holditch (2006) can be classified as a tight gas reservoir where a tight gas reservoir is "a reservoir that cannot be produced at economic flow rates nor recover economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores". In order to characterize the Unayzah Formation we used newly acquired geological and laboratory data obtained from detailed core description, facies, petrographic-and reservoir quality analysis. In a second step we translated these geological laboratory related data into rock types by integrating these data with MICP data considering pore throat geometries for a more accurate reservoir quality characterisation of the Unayzah Formation.
Even though coring of rocks is the best way to characterize reservoir and source rocks geologically and petrophysically, this method is considered expensive, having a relatively high cost per foot. Alternatively, side-wall cores and cuttings are widely used in reservoir characterization at a relatively low cost. However, this method has limitations related to cuttings bad physical conditions, size, mixed lithological and mineralogical characteristics which make the commonly used conventional evaluation methods not applicable. This study introduces a robust combination of digital and conventional core analysis methods to overcome these limitations and characterize reservoir and shale cuttings derived from two hydrocarbon-bearing formations in New Zealand.
Initially, all cuttings from both formations were screened based on their cutting sizes and later based on the visually observed textures using the stereomicroscope. This helped in selecting representative cuttings for the main identified textures. These cuttings were CT imaged at a resolution ranging from 40 to 4 microns/voxel resolution in order to confirm their rock textures and sedimentary structures for better characterization results. Next, mercury injection capillary pressure (MICP), X-ray diffraction (XRD), and petrographical analysis were conducted on all selected cuttings with different rock textures in order to understand the pore types, textural variations, diagenetic overprints and mineralogy of the cuttings samples. Then, they were scanned at optimum resolutions using Micro CT and 3D FIB-SEM microscopies. Finally, all acquired images were segmented digitally and 3D rock volumes were created. These volumes were used in computing porosity, permeability, formation factor resistivity (FRF) and poroperm trends digitally using numerical simulation techniques.
Conventional and digital rock analysis showed that the cuttings derived from the reservoir interval are composed of an argillaceous sandstone with a very good computed porosity (18% up to 31%) and permeability (30 to 200 mD). On the other hand, the cuttings derived from the shale source rock interval, which were predominately composed of clay minerals, have a computed porosity of 12% to 13% (mainly inorganic pores) and an absolute permeability in the range of 0.5 to 4 Micro-Darcy. The digital poroperm trend analysis identified distinct poroperm trends for each formation which helped in understanding their petrophysical aspects.
This integration between conventional and digital methods provided better geological and petrophysical understanding of both formations using a limited number of cuttings, less cost and time.
This section features industry or work-related photographs submitted by readers. Selected pictures will be published on the website. Be sure to provide your full name, job position, company name, picture location, and a caption for the picture. Recently, I had the opportunity to go onboard the FPSO Armada Claire. Another beautiful sunrise onboard SIEM Helix 2 NS-52, where i-Tech 7 has two remotely operated vehic...
Africa (Sub-Sahara) Oil was discovered at the Ekales-1 wildcat well located in Block 13T in northern Kenya. The well has a potential net pay of between 197 and 322 ft in the Auwerwer and Upper Lokone sandstone formations. Tullow (50%) operates 13T with partner Africa Oil (50%). The Mzia-3 appraisal well in Block 1 off Tanzania encountered a combined total of 183 ft of net pay in the Lower and Middle sands and confirmed reservoir quality in line with that seen in the Mzia-1 and Mzia-2 wells. Asia Pacific The Luba-1 offshore well on Brunei Block L was spudded. The well will evaluate the hydrocarbon potential of the Triple Junction structure. Serinus has a 90% interest in Block L, through indirect wholly owned subsidiaries Kulczyk Oil Brunei (40%) and AED SEA (operator, 50%).
Africa (Sub-Sahara) Gas was discovered at two separate levels in the Mronge-1 well in Block 2 offshore Tanzania. The discovery is estimated at between 2 and 3 Tcf of natural gas in place, bringing Block 2's estimated total in-place volumes up to 17 to 20 Tcf. Statoil (65%) operates the Block 2 license on behalf of Tanzania Petroleum Development Corporation, and partners with ExxonMobil Exploration and Production Tanzania (35%). Oil was discovered at the Agete-1 exploration well on Block 13T in northern Kenya. The well, drilled to a total depth of 1929 m, encountered 330 ft of net oil pay in good-quality sandstone reservoirs. Tullow Oil (50%) is the operator with partner Africa Oil (50%). Asia Pacific Indonesia announced plans to offer 27 oil and gas blocks in 2014 in regular tenders and direct offers.
Africa (Sub-Sahara) Drilling began on the Bamboo-1 well, located around 35 miles offshore Cameroon in the Ntem concession. The Bamboo prospect is a basin floor fan target within an Upper Cretaceous play. The well will be drilled to an estimated depth of 4200 m. Murphy Cameroon (50%) is the operator, with partner Sterling (50%). The Nene Marine 3 exploration well--located in the Marine XII block, which is around 17 km offshore Congo--encountered a wet gas and light oil accumulation in a presalt clastic sequence Eni (65%) operates the Marine XII block, with partners New Age (25%) and Société Nationale des Pétroles du Congo (10%). CNPC said PetroChina is now building a production facility capable of pumping 4 Bcm/yr.
Africa (Sub-Sahara) Eni finished a production test on its Minsala Marine 1 NFW well, located in Marine XII block, 35 km offshore The Republic of the Congo. During the test, the well delivered natural flow in excess of 5,000 B/D of 41 API crude and 14 MMcf/D of natural gas from a 37-m opened section of the discovery's 420-m column. Eni (65%) is operator, with state-owned partner SNPC (25%), and New Age (African Global Energy) Limited (10%). Asia Pacific CNOOC started natural gas production from the Panyu 34-1/35-1/35-2 project at the Pearl River Mouth basin in the South China Sea. Main production facilities for the three gas fields include one comprehensive platform, two sets of underwater production systems, and 13 producing wells. Two wells are producing a total of 21 MMcf/D of gas. The project is expected to reach peak production of 150 MMcf/D.
The Devonian-Mississippian STACK/SCOOP Play of the Oklahoma Anadarko Basin is a complex assemblage of tight carbonate and siliciclastic strata and an important oil and gas province. In the last decade, prolific drilling has demonstrated significant heterogeneity in the composition of oils produced from STACK/SCOOP reservoirs. This study discusses possible geoscientific explanations for the heterogeneity observed in produced oils and describes how source, maturation, and migration affect their composition.
Geochemical data from 136 produced oils across 12 counties from 4 producing reservoirs is reviewed. Calculated thermal maturity (Rc%) from alkylated polyaromatic compounds shows excellent agreement with oil thermal maturity increasing with increased depth. Oils produced from overpressured reservoirs exhibit a strong relationship between Rc% and Gas-Oil Ratio (GOR), while normal- to underpressured reservoirs exhibit GORs up to an order of magnitude higher at similar Rc%. Light hydrocarbons show that paraffinicity varies starkly with producing reservoir, suggesting compositional fractionation from diffusive migration through tight and argillaceous strata. Conversely, aromaticity varies geographically by Play Region, indicative of changing depositional environments and organic input across the basin. Isoprenoid and sesquiterpane biomarkers indicate all oils are generated by Type II or Type II/III mixed organic matter, but Springer Group reservoirs are charged by a highly argillaceous, non-Woodford source.
The Anadarko Basin is the deepest sedimentary basin in the cratonic interior of the North America with as much as 40,000 feet of Paleozoic sediments (Johnson, 1989). The Anadarko is an asymmetric basin with the deepest sediments bound against the Amarillo-Wichita Uplift to the southwest. The basin is elongated along its west-northwest axis and bound by the Nemaha Ridge to the east and the Anadarko shelf to the west and north.
In the last decade, drilling of Devonian-Mississippian strata along the margins of the basin have delineated one the continent's most successful petroleum resource plays. These areas are colloquially referred to as the
Determination of ideal horizontal targets for unconventional reservoirs often necessitates an understanding of the reservoir from the global tectonic to the sub-microscopic scale. When selecting a target zone, it is necessary to consider the abundance, composition, and delivery of sediment to basins; the production, preservation, and alteration of organic matter; and the diagenetic and structural modification of the stratigraphic section. Here, we focus on two sedimentologic phenomena common to the Marcellus Shale of the Appalachian Basin of southwestern Pennsylvania. Namely, we explore the strategy of targeting high organic carbon/biogenic silica facies and the challenges posed by encountering carbonate concretion horizons.
Geochemical observations including Si/Al and Si/Zr, and thin section and scanning electron microscopy indicate abundant recrystallized biogenic quartz cement in the Marcellus Shale. Burial models suggest that prior to the end of mechanical compaction; the Marcellus entered the oil window, and presumably began generating organic matter-hosted porosity at a depth of ~1200m. Notably, at similar organic carbon content, samples with elevated biogenic silica yield higher porosity and permeability. These observations suggest that biogenic quartz may play a role in the deliverability of hydrocarbons by providing a compaction resistant framework conducive to the preservation of organic matter-hosted pores and pore throats. Further, biogenic quartz-rich facies demonstrate increased rates of penetration allowing for more efficient drilling of laterals.
However, carbonate concretions encountered while drilling horizontal Marcellus Shale wells negatively affect drilling operations by reducing drilling rates, damaging bits, and requiring excessive steering corrections to penetrate or extricate the bit from the horizon. Carbonate concretions form by the anaerobic oxidation of methane in a narrow zone perhaps just a few meters below the seafloor. Crucial to this mechanism is a slowing or pause in sedimentation rate that would have held the zone of carbonate precipitation at a fixed depth long enough for concretions to grow. Using this model, we attempt to predict the size and location of concretions to avoid encountering them while drilling. Field observations of Upper Devonian shale-hosted concretion dimensions suggest that Marcellus-hosted concretions up to three feet in length are possible. Hiatuses in sedimentation and potential concretion horizons were predicted using uranium to organic carbon ratios. The attachment of uranium to organic carbon macerals occurs across the sediment-water interface. Therefore, an increase in the abundance of uranium per unit organic carbon indicates a cessation in sedimentation and the potential for concretion growth. Indeed, when comparing well log response to core, uranium to organic carbon excursions predicted the location of two concretion horizons.