E a r t h A n d E n v i r o n m e n t a l S c i e n c e s , B a h r i a U n i v e r s i t y , k a r a c h i

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PROPERTIES OF SOURCE ROCKS OF PAKISTAN

TABLE OF CONTENTS 1. Introduction................................................................................................................3

2. Source Rocks Of Pakistan.............................................................................................3

2.1 Overview........................................................................................................................3

3. Upper Indus Basin.......................................................................................................4

3.1 Patala Formation............................................................................................................4

4. Middle and Lower Indus Basin.....................................................................................4

4.1 Sember Formation Shales ..............................................................................................4

4.2 Kirthar Fold Belt.............................................................................................................5

4.3 Sulaiman Fold Belt......................................................................................................... 5

5. Baluchistan Basin........................................................................................................5

5.1 Rakhshani Formation.....................................................................................................5

5.2 Kharan Limestone.......................................................................................................... 5

6. Pishin Basin.................................................................................................................5

6.1 Dark Grey to Black Shales...............................................................................................5

6.2 Black Limestone.............................................................................................................6

7. References...................................................................................................................6

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PROPERTIES OF SOURCE ROCKS OF PAKISTAN

1. INTRODUCTION

In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including deep water marine, lacustrine and deltaic. 

A rock rich in organic matter, which if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Preservation of organic matter without degradation is critical to creating a good source rock, and necessary for a complete petroleum system. Under the right conditions, source rocks may also be reservoir rocks, as in the case of shale gas reservoirs.

A source rock is formed by the:

Sedimentation in areas of high organic productivity.

Preservation of organic matter in sediment.

2. SOURCE ROCKS OF PAKISTAN

2.1. OVERVIEW

In Potwar Patala shale (Paleocene) is the source rock. Salt Range Fm (Neoproterozoic) also reportedly contains random high TOC pockets, but salt being not permeable would inhibit the movement of generated hydrocarbons. In rest of Pakistan, except Baluchistan Basin (west of Ornach-Nal-Chaman faults), Sembar shales (Lower Cretaceous) are considered as the source rock. Habib Rahi (Eocene) reportedly contains some higher TOC in Sulaiman Range and might be mature in the foredeep, but not yet proven.

Any depression can act as a source rock, enabling it to generate hydrocarbons. There are no names as such. In Potwar the main kitchen is Soan syncline, and other depressions between the anticlines may also act as kitchen, under the required temperature-pressure conditions. The Kirthar and Suliaman foredeeps are considered to feed the traps on their either side. Sui field has its own kitchen right beneath the Sui dome, which fed the surrounding structures also. Badin area represents horst-graben structural geometry, where grabens would act as kitchen.

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Potential source rocks ranging in age from the Cambrian to Eocene, are fairly well distributed in the sedimentary basins of Pakistan. These source rocks differ in the type of organic matter, abundance and maturity in the different basins and have generated oil, gas and condensate. Pakistan has been divided into four basins for discussing the source rock distribution and geothermal gradients as under:

Upper Indus Basin Lower Indus Basin Baluchistan Basin Pishin Basin

3. UPPER INDUS BASIN

The source rock in Kohat area is probably Chichali formation (Lower Cretaceous), and in Potwar, Patala shale (Paleocene) is considered as the source rock.

3.1. PATALA FORMATION (EARLY PALEOCENE AGE)

Patala Formation of Early Paleocene age is the primary source rock in the basin responsible for hydrocarbon generation. Some them working on Jurassic have suggested that Jurassic shales of Datta Formation also have source potential for generation of lighter hydrocarbon and even for gas.

4. MIDDLE AND LOWER INDUS BASINS

In Middle Indus and Lower Indus basins the formations found are as follows:

4.1. SEMBER FORMATIONS SHALES (EARLY CRETACEOUS)

Early Cretaceous Sembar Formations shales have been widely accepted as an essential source rock for oil and gas. However, fields in Central Indus basin (Mari and Sui, Uch) may have been charged from source rocks other than Sembar most probably in Eocene.

There are some other sources such as Ranikotshales, Ghazijshales and coals and shales of Nari Formation (both of paleogene age). However, it needs more studies to conduct for the evaluation on regional level and also to field level to authenticate it.

Lower Indus basin can further be divided into two fold belt basins:

Kirthar Fold Belt Basin Sulaiman Fold Belt Basin

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4.2. KIRTHAR FOLD BELT BASIN

The Paleocene (mainly Ranikot Formation) is considered as source horizon with TOC contents up to 1 %. Sembar & Goru formations of Lower Cretaceous age are proven source rocks in the Lower Indus Platform Basin.

These are considered as potential source rocks in the Kirthar Foldbelt. Excellent oil prone source rocks are recorded in the Lower Cretaceous section in the east (Sann-1), which spans the top half of the main oil window.

4.3. SULAIMAN FOLD BELT BASIN

Source rocks exist in Cretaceous (Sembar & Goru formations), Paleocene (Ranikot Formation) and Eocene (Habib Rahi) stratigraphic successions. However Sembar Formation of early Cretaceous is considered to be the dominant source rock. The source rock maturation started in Cretaceous and reached peak gas generation during Eocene-Miocene. The trap formation coincides with the peak stages of gas generations.

5. BALUCHISTAN BASIN

The potential source rocks of the area are Rakhshani Formation of Paleocene age and Early to Middle Eocene Kharan Limestone.

5.1. RAKHSHANI FORMATION

Rakhshani Formation of Paleocene age contains source horizons in shale layers. Its source potential in term of TOC is moderate to good.

5.2. KHARAN LIMESTONE

Kharan Limestone has a fetid odor and the basal part of the formation, which is composed of wackstone, deposited in an environment favorable for the accumulation and preservation of organic matter.

These source rocks are predicted to be presently mid to late mature for oil generation and mature for gas generation.

6. PISHIN BASIN

Two types of source rock facies are identified in three distinct horizons.

6.1. DARK GRAY TO BLACK SHALES

The first type of source rock is dark gray to black shale associated with transgressive system. The shale is considered to be linked with basal black shales in the subsurface and may be termed

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as basal black shale. This shows similarity in facies and organic richness with the upper part of Paleocene/Eocene Patala shale of the oil producing Potwar Depression. Further northwestward in the center of the basin the shale is supposed to carry well developed condensed section hence has better source rock parameters. The gray shale associated with the low stand of sequence. These source rocks are distributed throughout the basin.

6.2. BLACK LIMESTONEThe second type of source rock is black limestone. This limestone possesses fairly high TOC value and resembles in physical character and microfacies with Habib Rahi Limestone (oil shale) of Sulaiman Range having very high TOC. The organic rich limestone is restricted to the northwestern deeper part of the basin. Laboratory support proved that these horizons have fair to high organic content.

7. REFERENCES

http://en.allexperts.com/q/Geology-1359/2011/1/stratigraphic-column-Pakistan-1.htm

www.glossary.oilfield.slb.com/Display.cfm?Term= source %20 rock

http://www.fallsoftheohio.org/SourceRockforPetroleum.html

http://www.docstoc.com/docs/71832724/Petroleum-Geology-of-Pakistan-By-Dr-Muhammad-Mujtaba

http://ebookbrowse.com/petroleum-geology-of-pakistan-by-i-b-qadri-akbar-ali-asif-pdf-d179084626

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