Geologic Background edit
Tectonic Setting edit
The South Pars/North Dome Gas Condensate Field is located in the Persian Gulf, and is part of the NNE – SSW trending Qatar Arch. The Qatar Arch is located in the interior of the Arabian Plate and is bounded by the Zagros Folded Belt at the north and northeast. The major structural features of the region were formed during two main tectonic phases. The first tectonic phase was the Amar Collision, approximately 640-620 Ma, that occurred along the North-South trending Amar weak zone in the Arabian Shield[1]. This event lead to the consolidation of the Arabian Plate, as well as formation of the Qatar-Fars High and the Ghawar High. The second tectonic phase corresponds to the Najd Rift System approximately 570-530 Ma[1]. This was responsible for the rejuvenation of the north-south structural development of the sedimentary cover and the distribution of the hydrocarbon reservoirs in the area. This extensional phase lead to the development of the basins in the region, and the resulting geologic deposits were uniform clastic continental and shallow-marine sedimentation on the stable passive margin of northeastern Gondwana.
The Persian Gulf Basin is comprised of several NW-SE trending geotectonic units including the Arabian Platform. The basin is also comprised of a zone of marginal troughs, such as the Zagros Fold Belt, and it is limited to the northeast by the Main Zagros Reverse Fault. The Qatar–Kazerun lineament, also known as the Qatar Arch, is an important north–south structure which divided the Persian Gulf Basin into two troughs, the ESE and the WNW sub-basins[2].
Stratigraphy & Sedimentology edit
There are four major tectono-sedimentary units in the Qatar Arch. The Post-Precambrian to Pre-Permian units are the oldest units. It is composed of the Precambrian Arabin Basement, the Lower Silurian Sarchahan Formation, and the Devonian Zakeen Formation. During this time, the Persian Gulf Basin was a zone of steady subsidence[2].
The Permo-Triassic Unit is composed of the Early Permian Faraghan Formation, Late Permian Dalan Formation, Early Triassic Kangan Formation, and Late Triassic Dashtak Formation. These formations were deposited in the newly opened Neotethys Ocean. The Aghar Shales at the base of the Dashtak Formation acts as a regional seal for the Qatar Arch hydrocarbon system[2].
The Jurassic-Cretaceous marine carbonate units were deposited when vast epicontinental sea developed this time period[3]. During the Triassic to Jurassic transition there was a period of erosion and non-deposition, likely caused by structural uplift combined with a lowstand sea level in the Qatar Arch. This section is composed of the Early Jurassic Neyriz Formation, the Jurassic to Early Cretaceous Khami Group, and the Cretaceous Bangestan Group[2].
The Late Cretaceous to Recent Rock Units contain the uppermost sequences of rocks deposited after the Zagros Orogeny. These carbonate depositions likely resulted from the progressive closure of the Neotethys Ocean in the region. It is composed of the Paleocene to Eocene Sachun/Jahrum Formations, the Oligocene Asmari Formation, and the Miocene to recent Gachsaran/Mishan Formations[2].
There are five main depositional facies in the Khuff Formation, supratidal/sabkha, intertidal, lagoon, shoal, and shallow shelf. All of these were deposited in a carbonate ramp. The sabkha facies is dominated by anhydritic dolomitic mudstone. There are massive deposits of nodular anhydrite, as well as nodular masses of dolomite within the mudstone. The sabkha facies also contains oolithic-peloidal limestones that are partially dolomitized and anhydrite-cemented[4]. The intertidal facies are dominated by massive and homogeneous dolomite lithology with mudstone texture[2]. There are microbial laminates with fenestrate and laminated structures, and fenestral dolomitic mudstone and wackestones with ostracod shells present. The lagoon facies are dominated by peloidal-oolithic grainstone, packstones, bioclastic dolomitic mudstones, and in rare cases bioclastic peloidal wackestones. The lagoon facies is highly bioturbated with gastropods, small foraminifers, algae, and ostracods[4]. The shoal facies is dominated by oolite-rich dolomitic grainstones and packstones, with some foraminiferal peloidals present. Massive structures, cross bedding, or grain grading and orientation is present[4]. The shallow shelf is dominated by well laminated dolomitic mudstone with open marine fauna present[4].
Structure and Trapping edit
The gas and condensate in this petroleum system are trapped in one of three different ways. The reactivated basement fault blocks from the Qatar Arch created north-trending large, gentle anticlines. Salt Domes have formed in the region from halokinesis, or salt tectonics. The Zagros folding created NW–SE trending structural traps. These three different traps have led to massive reservoirs of gas and condensate in this petroleum system[2]. The extensional phases caused by the reactivation of the fault systems lead to the formation of numerous rift basins in the region. These rift basins led to the formation of anticlines and structural traps from horst uplift and faulting. Salt diapir uplift also resulted from the reactivation[5].
Salt is mechanically weak and and flows like a fluid, and this fluid rheology and incompressibility make the salt unstable under a wide range of geologic conditions. The two major driving forces of salt tectonics are differential loading and buoyancy. Salt tectonism is closely tied to regional deformation due to its weak nature[6]. The Precambrian basement in the Arabian Platform is unstable and has been repeatedly reactivated during the Phanerozoic. The reactivation of the Arabian Platform is thought to have been responsible for triggering salt diapir uplift[7]. Due to the extension of the Qatar Arch, salt diapirs rose up graben axes taking advantage of the space created by the thinning of fault blocks.
The main gas and condensate reservoirs are found in the thick limestone and dolomite sequences of the Permian Dalan and Triassic Kangan Formations, as well as some in the Khuff carbonates. The anhydrite and shale successions of the Triassic Dashtak Formation provides a seal for the reservoirs[2]. The impermeable units of anhydrites, carbonates, and shales in the Khuff Formation are also part of the major regional seal.
Source Rock edit
The hydrocarbon source rock for the gas in the Khuff reservoir is the organic-rich Lower Silurian Hot Shale. In the Arabian Gulf region geochemical analysis of the Lower Silurian shales show fair to good source rock potential that are sufficiently mature and rich in organic matter[4]. The Lower Silurian shales are thought to be the source rock for hydrocarbon reservoirs across the Arabian Gulf[5]. The Khuff Carbonates were thought to possibly be the source rock, but the vitrinite reflectance, maximum TOC values, and pyrolysis yield calculated from geochemical analyses indicates poor source-rock potential[4]. This further increases the likelihood that the underlying Silurian shales are the source.
Reservoirs edit
There are four reservoir units in the Upper Dalan–Kangan strata in the South Pars field, K1, K2, K3, and K4. K4 and K3 are located in the Upper Dalan Member of the Dalan Formation, which is Late Permian in age. K2 and K1 are located in the Kangan Formation which is Early Triassic in age[2]. K4 was the deepest interval and has an average gross thickness of 165 m. K4 is composed of 55-60% dolomitic carbonate that overlies the Nar anhydrite member. K4 has the highest average porosity of the units based on core analysis. K3 has an average gross thickness of 121 m[2]. K3 is composed of 70% dolomite, with thick intervals of anhydrite and anhydritic carbonate which is identified as the permeability barrier which separates K3 from K4. K2 is about 42 m thick and is composed of limestone. The porosity and permeability of this layer shows rapid vertical variations. Porosity can range from 0-35% and permeability can range from 0 to 1000 mD. There is an unconformity between K3 and K2, with the base of K2 being defined by the Permo-Triassic boundary. Thick layers of tight thrombolite facies first appear in this unit[2]. The K1 interval is about 111 m thick. It is composed of 70-80% dolomite with thick anhydrite and anhydritic carbonates separating the top of K2 and the basal of K1. The K1 interval in general has a higher porosity but lower permeability[2].
Notes edit
- ^ a b "Diagenetic controlled reservoir quality of South Pars gas field, an integrated approach". Comptes Rendus Geoscience. 343 (1): 55–71. 2011-01-01. doi:10.1016/j.crte.2010.10.004. ISSN 1631-0713.
- ^ a b c d e f g h i j k l Esrafili-Dizaji, B.; Rahimpour-Bonab, H. (2009-10-26). "Effects of depositional and diagenetic characteristics on carbonate reservoir quality: a case study from the South Pars gas field in the Persian Gulf". Petroleum Geoscience. 15 (4): 325–344. doi:10.1144/1354-079309-817. ISSN 1354-0793.
- ^ Konyuhov, A. I.; Maleki, B. (2006-07-01). "The Persian Gulf Basin: Geological history, sedimentary formations, and petroleum potential". Lithology and Mineral Resources. 41 (4): 344–361. doi:10.1134/S0024490206040055. ISSN 1608-3229.
- ^ a b c d e f Alsharhan, Abdulrahman S. (2006-07-01). "Sedimentological character and hydrocarbon parameters of the Middle Permian to Early Triassic Khuff Formation, United Arab Emirates". GeoArabia. 11 (3): 121–158. ISSN 1025-6059.
- ^ a b Pollastro, Richard M. (2003). "Total petroleum systems of the Paleozoic and Jurassic, Greater Ghawar Uplift and adjoining provinces of central Saudi Arabia and northern Arabian-Persian Gulf". Reston, VA. doi:10.3133/b2202h.
{{cite journal}}: Cite journal requires|journal=(help) - ^ Hudec, Michael R.; Jackson, Martin P. A. (2007-05-01). "Terra infirma: Understanding salt tectonics". Earth-Science Reviews. 82 (1): 1–28. doi:10.1016/j.earscirev.2007.01.001. ISSN 0012-8252.
- ^ Perotti, C.R.; Carruba, S.; Rinaldi, M.; Bertozzi, G.; Feltre, L.; Rahimi, M. (2011-08-09), Schattner, Uri (ed.), "The Qatar–South Fars Arch Development (Arabian Platform, Persian Gulf): Insights from Seismic Interpretation and Analogue Modelling", New Frontiers in Tectonic Research - At the Midst of Plate Convergence, InTech, doi:10.5772/20299, ISBN 978-953-307-594-5, retrieved 2020-11-29