Класс Месторождения: Крупное
Тип Месторождения: Газоконденсатное
Местоположение: Суша
Местность:
Стадия разработки: Добыча
Год открытия: 1952
Источник информации:
Метод открытия:
Площадь: 177.84 км²
Sui Gas Field
The Sui Gas Field is located in the Balochistan Province about 50 miles north-north-east of the town of Khairpur. The field was discovered, by Pakistan Petroleum Limited in 1952, as a result of drilling over an exposed anticline. It was the first major gas discovery in Pakistan and remains the largest with reserves over 9.0 Tscf (0.255 Tscm). Commercial production commenced in 1955 at a rate of about 35 MMscfd (1.0 MMscmd). As the demand for the gas increased, the production rate was enhanced by development drilling. A total of 86 wells have been drilled on the structure so far. By mid 80s Sui was capable of delivering at a maximum rate of about 850 MMscfd (24 million cubic meters per day). The field is producing gas from two reservoirs i.e. Sui Main Limestone (SML) and Sui Upper Limestone (SUL). A gradual decrease in the pressure of the SML Reservoir necessitated installation of compression facilities to maintain the flow rate at contractual pressure. The field compression facilities were commissioned in 1986. The compression facilities for the SUL have been commissioned during 1997-98. It is currently planned to enhance the compression capacity at Sui field by installing second stage compression to increase the plateau period and the ultimate gas recovery. The gas discovery at Sui has played a major role in the development of all phases of the natural gas infrastructure in Pakistan and has provided an essential resource for the industrial growth.
Figure 1. Location map showing the study area and Eocene outcrops across the lower Indus basin. The outcrops are not shown in the TransIndus Range (TIR) and Potwar Plateau (Eocene outcrops based on the geologic map of Pakistan, 1:2,000,000; modified from Bakr and Jackson, 1964). SML = Sui Main Limestone.
STRATIGRAPHY
The Sui structure is situated in the Sulaiman Sub-basin of the Indus Basin (Figure-2). It is part of the first set of folds emerging from the Indus flat planes west of the Indus River. The sediments of the Siwalik Formation, ranging in age from Miocene to Pleistocene are exposed in the region. These late Tertiary sandstones, clays and conglomerates were folded and faulted during the Himalayan orogenic episode in an arcuate belt along the eastern margin of the Sibi re-entrant (Sohail and Abid, 1983).
A typical stratigraphic column encountered in Sui Field is shown in Figure-3. It is based on the formations encountered in the discovery well, SUI-1. The Siwalik rocks overlie the shales and sandstones of the Gaj-Nari formations, with an unconformable contact. The Gaj-Nari formations are from Miocene to Oligocene in age. These formations show a thinning towards the crest of the dome from all directions.
Another unconfomity separates the Gaj-Nari formations from the underlying Kirthar Formation. The Kirthar Formation includes Dozkushtak, Pirkoh and Habib Rahi Limestones. These limestones are separated by shale beds of different thicknesses. Habib Rahi Limestone is an important gas reservoir in the region and is productive in different fields. At Sui it is gas bearing, however it does not contain good quality gas.
The Laki Formation of Lower Eocene Age, which underlies the Kirthar Formation includes the SUL, Ghazij Shale and the SML. SUL consists mainly of limestone with interbedded shales, while the SML is predominantly limestone with marl and thin shale marker beds in the basal part. The two reservoirs in the Sui Field have both matrix and fracture porosity. The SML is the thickest in Sui area, attaining thickness of over 640 meters. Away from the Sui depocenter the thickness decreases with a corresponding increase in the thickness of Ghazij shale.
To the north, the SML is not encountered in Pirkoh and in the south the limit of SML as a reservoir can be placed south of Khairpur area. The lower most part of the SML is considered to be of Paleocene Age by some workers based on faunal evidence.
The Paleocene age Ranikot Formation is mainly composed of shales with some limestone and quartzose sandstone beds.
The underlying Cretaceous Section is separated from the overlying Paleocene by the worldwide TertiaryCretaceous unconformity. The complete Cretaceous sequence from Pab Formation to Sembar Formation is present in the Sui area. The recent encouraging discoveries within the Cretaceous in the Middle Indus Basin have increased the prospectivity of the Pab sandstone, Moghalkot limestones and sands within the Lower Goru Formation. All these potential reservoirs are being targeted in the Sui Deep well presently being drilled.
Below the Cretaceous, the Chiltan limestone of Jurassic age is another potential reservoir in the Sui area. The Jurassic is underlain by rocks of older age.
STRUCTURE
The Sui structure is a very large, gentle, ellipsoidal anticline exposed at surface, marked by rocky outcrops forming low hills and cliffs. The major axis of the field at surface is about 55 Km long and is in the east-west direction. The surface dips are gentle in the range of 1° to 4°.
Lower Siwalik shales and clays are exposed at the crest of the structure. The conglomerates from the Upper Siwaliks form a ring of steep escarpments around the crest of the structure (Figure-4).
The discovery well Sui-1 was drilled based on surface geology. The drilling of the appraisal wells 2 to 5 confirmed that the subsurface structure is similar to that mapped by the surface geological evidence. In order to fully map the subsurface structure at the levels of SML and SUL, a detailed vibroseis seismic survey was conducted in 1975. The surface grid coverage was from about 3.2 to 4.0 Km. The results of this seismic surveyfurther confirmed the earlier interpretation of the shape and size of the structure. The structure contour map based on the seismic interpretation and well data is shown in Figure-5.
One of the seismic lines running east-west through the central part of the field is shown in Figure-6.
On the seismic section the top Habib Rahi Limestone is picked below 0.5 seconds, while top SUL and top SML reflectors are picked in the range of 0.9 to 1.0 seconds.
Top Chiltan is picked around 2.6 seconds. The seismic survey of 1975 was optimized to map the Sui Upper and Sui Main limestones and did not provide the desired results for deeper horizons. Another seismic survey to better image the deeper horizons was completed during the first quarter of 1997. The structural interpretation and seismic sequence stratigraphy based on the new data confirmed that the Sui structure is more or less conformable to the deeper levels and provided further evidence of distribution of Lower Goru sands in the Sui area. To determine the aerial extent of the Sui structure, integration / interpretation of the 1975 seismic data with the 1997 data is currently plarmed and as a first step, reprocessing of the 1975 data has been completed.
Surface geological mapping and aerial photographs have revealed the presence of a number of mostly north-south trending faults. Most of these have very small displacement. The seismic interpretations and the drilling evidence show little subsurface displacement associated with these surface features.
The changes in thicknesses of the Gaj-Nari formations towards the crest of the structure were most likely caused by the formation of a gentle dome in post-Eocene/PreMiocene time. The surface of this dome was formed by the Dozkushtak limestone which was exposed to submarine or subaerial erosion for considerable time. It appears that there was also some movement during the deposition of Gaj-Nari and Siwaliks. The main movements were in Plio-Pleistocene time with the folding of the Siwaliks about the earlier axis.
Data source: SUI GAS FIELD - A CASE HISTORY. Suhail Qadeer and Nusrat K. Siddiqui. 2022
Следующее Месторождение: Mithrao