Assalam o aliqum sir my questions is 1 what are the essential elements of reservoir rock please discuss? 2 history of Dhulian oil field? 3 can you explain two main functions of traps? 4 Is there any chance to discover any Giant hydrocarbon discovery in Indus basin I will be very thankfull
Thank you very much for sending me your volley of question and I am glad to respond you all.
Reservoir rock as you know is one of the essential element of Petroleum system where oil, gas and water stored. Consider reservoir rock as a house where people live; they need rooms, doors larger and as many between the rooms are better so you can easily walk around, larger the room means more space for your household goods and furniture to be stored, multi storied house means more people can live comfortably in a smaller surface area house whereas single story house would be better to have a house covering larger surface area. Considering the example I have given; in any given reservoirs Pores are you rooms, doors are the permeability and multi-story or multi stacked reservoirs means more space for fluids, surface area is the total drainage areas of the trapped reservoir.
Porosity is an amount or volume of pores or voids in a rock. Pores can be in between grains or within rock particles or fractures. Porosity is generally expressed in fraction or in Percent. Different rocks have different range of porosity; sandstone and limestone may have as high as 50 to 60% if poorly cemented and uncompacted but in general the average porosity range in a good reservoirs in large fields between 20 to 30%. Any number lower than 8 to12% will be considered a tight reservoir.
Permeability is a measure of the ease that allows fluids to move freely through a porous rock. As a general rule higher the porosity rock should have higher permeability but depends on the degree of pores connectivity. Remember the doors in your rooms; wider and open and as many between rooms you can walk across the rooms easily. Rooms without doors means no entry and exit point in the rooms and whoever have trapped in there cannot communicate with a person living next room. This rule exactly applies to the oilfield reservoirs as well. Fracing (or fracturing through pumping water or acid in the well bore from the surface pumps) is the process in which these doors are created or enhancing the permeability of the reservoir to get maximum oil even from the isolated pores.
Another aspect is the Net to Gross ratio (NTG) between the total reservoir thickness and the net pay thickness. Higher the NTG would be better but it depends on how actually Net pay in thick. NTG is usually expressed in fraction or in percent.
About Dhulian Field; read my full paper. Abstract is as follows and availaible in the link I have added at the end of the abstract;
The Dhulian oilfield is located in the central part of the Potwar Plateau, about 90 km south-west of Islamabad. A.W.Wynne first mapped the surface structure in 1877 as a broad east-west trending, doubly plunging anticline. In 1918, the Attock Oil Company Limited started exploration on the surface structure. First commercial production was obtained in 1937 from the fractured shelf carbonates of Eocene Chorgali-Sakesar formations. First oil discovery in Pakistan from Paleocene reservoir was made during 1951 in Dhulian. In 1960 Pakistan's first Jurassic oil-bearing reservoir was also discovered in the Ohulian-39 well. A total of 49 wells were drilled till1966. The average well depth in the field is about 8,500 feet, but Dhulian 43, the deepest well, was drilled to a depth of 12,428 feet into Infra-Cambrian; the Permian and Cambrian objectives were water bearing.
In 1993 POL acquired seismic data which revealed that in the subsurface the Dhulian structure is a broad asymmetric compressional fold with faulted compartments. It is salt cored, with a wrench-induced major thrust to the north separating it from the Khaur pop-up stru cture. The trapping mechanism for the Tertiary reservoirs is a four-way dip closure, whereas the Jurassic oil is trapped by a combination of structure and reservoir pinchout. The Kuldana Formation (red clay) forms the top seal for the Eocene reservoir, whereas the Nammal and Datta variegated sh ales are the cap rocks for the Paleocene and Jurassic respectively. The Eocene marine carbonates and shales and the black marine shales of the Paleocene Patala Formation are believed to be the source rocks for the oil in Dhulian. The Paleocene and Jurassic reservoirs appear to be in pressure communication.
However, many wells, including Dhulian-46, produced oil from deeper Jurassic sand, while shallower Paleocene limestone in the same well produced only water, implying non-communication. Dhulian has an active water drive in the Eocene reservoir, whereas the principal drive mechanism in the Paleocene and Jurassic reservoirs is solution gas and partial water drive.
The Dhulian field has produced a total of approximately 41 mmbbls of oil and 199 beŁ of solution gas. The 500 feet thick Eocene shelf carbonates has produced 7.7 mmbbls of 28°-32° API oil with initial maximum reservoir pressure of 5960 psi. The main oil production of 21 mmbbls was obtained from the 450 feet thick Paleocene shelf carbonate and 12 mmbbls of oil has been produced from the 70 feet thick basal Jurassic Datta sandstone. The Palaecene and Jurassic reservoirs gradually watered out and the flow ceased completely in 1983. Prior to the initiation of an enhanced hydrocarbon recovery project in late 1997, daily production from the Eocene was only about 18 b / d oil and 256 b / d water. After the recent completion of the Dhulian-17 and -39 workovers, oil and gas production from the field has increased significantly.
Structural, Stratigraphic and Structural-stratigraphic combination are the commonly known traps. Any traps need to have closure which should be tightly packed by overlying, underlying and lateral seal rock that prevent hydrocarbon from further migration or leaking from the trap.
In Indus basin the possibility for a discovery of the giant field is very rare but exceptions cannot be over ruled. However, based on the present knowledge and trend of hydrocarbon discoveries it is less likely that any field larger than Mari and Miano-Sawan gas field or any field larger than Khaskeli and Tando Alam can be found from any of the known and established plays and reservoirs
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