SOURCE ROCK

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. Oil shale can be
regarded as an organic-rich but immature source rock from which little or no oil has been
generated and expelled. Subsurface source rock mapping methodologies make it possible to
identify likely zones of petroleum occurrence in sedimentary basins as well as shale gas plays.

TYPES OF SOURCE ROCK

Source rocks are classified from the types of kerogen that they contain, which in turn
governs the type of hydrocarbons that will be generated.

Type I source rocks are formed from algal remains deposited under anoxic conditions in
deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep
burial.

Type II source rocks are formed from marine planktonic and bacterial remains preserved
under anoxic conditions in marine environments: they produce both oil and gas when thermally
cracked during deep burial.

Type III source rocks are formed from terrestrial plant material that has been
decomposed by bacteria and fungi under oxic or sub-oxic conditions: they tend to generate
mostly gas with associated light oils when thermally cracked during deep burial. Most coals and
coaly shales are generally Type 3 source rocks.
EXAMPKES OF SOURCE ROCK FROM WORLD

Certain source rocks are referred to as "world class", meaning that they are not only of very high
quality but are also thick and of wide geographical distribution. Examples include:

 Middle Devonian to lower Mississippian widespread marine anoxic oil and gas source
beds in the Mid-Continent and Appalachia areas of North America: (e.g. the Bakken
Formation of the Williston Basin, the Antrim Shale of the Michigan Basin, the Marcellus
Shale of the Appalachian Basin).
 Kimmeridge Clay – This upper Jurassic marine mudstone or its stratigraphic equivalents
generated most of the oil found in the North Sea and the Norwegian Sea.
 La Luna Formation – This Late Cretaceous (mostly Turonian) formation generated most
of the oil in northwestern Venezuela.
 Late Carboniferous coals – Coals of this age generated most of the gas in the southern
North Sea, the Netherlands Basin and the northwest German Basin.
 Hanifa Formation – This upper Jurassic laminated carbonate-rich unit has sourced the oil
in the giant Ghawar field in Saudi Arabia

SOURCE ROCK EXAMPLES FORM PAKISTAN

Pakistan consist of basins namely

ONSHORE

1. Upper Indus Basis

2. Middle Indus Basin

3. Lower Indus Basin

4. Baluchistan Basin

5. offshore Idus basin

 Basins in Pakistan

1. Upper Indus Basin

Source Rocks of Upper Indus Basin

Main source rock are

i. In Potowar, Patala Shale (Paleocene) Primary source rock
ii. In Kohat, probably Chichali Formation (Lower cretaceous) considered as source rock
And other sources are
iii. Mianwali Formation (Lower Triassic)
iv. Dondot Formation (Early Permian)
v. Sarddhai Formation (Early Permian)
Reservoir Rocks of Upper Indus basin
a. Warcha Formation (Early Permian)
b. Wargal formation (Permian)
c. Lokhart Formation (Paleocene)
d. Chorgali Formation (Eocoene)
e. Muree Formation (Miocene)
Traps/Structures of Upper Indus Basin:
In Kohat-Potwar area (Upper Indus) we mainly have structural traps. Fault-bounded pop-
up structures are common in the eastern Potwar (like Adhi). Fault-bounded with dip closure on
three sides are encountered towards west.

Source Rocks of Lower Indus Basin

2. Lower Indus Basin

Main Source Rocks of Lower Indus basin
a. Sembar Shale Formation (Early cretaceous) widely accepted as main source rock
b. Ranikot Shale (Palaeocene)
Other source rocks are
c. Ghazji Shale Formation (Eocence)
d. Coals and Shales of Nari Formation (Paleocene)
e. Mughal kot Formation (Campanian to Early Maestrichtian)

Reservoir Rocks of Lower Indus basin

Main reservoirs of oil and gas

a. Pab sandstone (Maestrichtian) and
b. Lower Guru Formation (lower cretaceous)
Other reservoirs are
c. Bara Formation (Middle Paleocene)
d. Laki Limestone Formation (Early Eocene)
Traps/Structures of Lower Indus Basin
In Lower Indus basin, we have broad domes in Kirthar fold belt, while dominantly horst-
graben geometry traps are productive in Badin area; east of Badin area updip pinchouts may
provide stratigraphic plays.

3. Middle Indus Basi

Dominant/ main Source Rocks

a. Middle Guru Formation (Cretaceous)

b. Sembar Formation (Cretaceous)

And other sources are

c. Khirthar Formation (Middle Eocene to Oligocene)
d. Anjira Member (Early-Middle Jurassic)

Reservoir Rocks of Middle Indus Basin

a. Sui Main Limestone Formation (Eocene to Paleocene)

b. Habib Rahi Limestone Formation (Eocene)
c. Lower guru Formation (Cretaceous)
d. Chiltan Formation (Middle Jurassic)
Traps/Structures of Middle indus Basin
In Middle Indus region the Sulaiman Range offers relatively narrow but long N-S oriented
anticlines (Zindapir etc). In Sulaiman Lobe region we generally have broad domes like Sui.
Kandhkot, Qadirpur, Mari etc. Along the eastern flank of Jcobabad-Khairpur high we do find
stratigraphic-cum-structural traps. While in Punjab Platform the possibility of up-dip pinchouts
towards east exist; horst-graben geometry is also common.
Source Rocks of Middle Indus Basin

4. Baluchistan Basin

Sorce Rocks of Balochistan Basin

a. Rakshani Formation (Paleocoene)

b. Kharan Lime stone (Early to middle eocene)
Other sources are
c. Hoshab Formation (Late Oligocene to Early Miocene)
d. Panjgur Formation (Miocene)
e. Parkana mudstone Formation

Reservoir Rocks of Balochistan Basin

a. Talar Sandstone

Traps/Structures of Balochistan Basin

Moin raza khan article 10385 (2012), the result of fault plane solutions ndicates the rifting
nature of basins indicating the probability of tlted fault block traps
Structure style in nausherwani block Balochistan is associated with transpressonal
tectonics, thrusted anticline, pop-up and flower structures. Closures are expected over these
structures of these considerable size, which can provide trapping mechanism to mgerating
hydrocarbons.
Intra-formational shale of rakshani, kharan and nauroz formation have potential to act as seal.
Based on tectonics the trap type can vary highly complex to simple. Area adjacent to chamman
fault system, highly complex structures are expected in that area, further west of chamman fault in
Mashkeland Dalbandn troughs, less complex thrusted anticlines, pop-ups and fault propagation
folds are expected

5. Pishin Basin

Sorce Rocks of Pishin Basin

a. Nissai Formation (Upper Eocene)
b. Murgha Faqir Zai formation (Upper Eocene)
Reservoir Rock of Pishin Basin
a. Khojak Sandstone (Oligocene)

MAURATION OF SOURCE ROCK

With increasing burial by later sediments and increase in temperature, the kerogen within
the rock begins to break down. This thermal degradation or cracking releases shorter chain
hydrocarbons from the original large and complex molecules occurring in the kerogen.

The hydrocarbons generated from thermally mature source rock are first expelled, along with
other pore fluids, due to the effects of internal source rock over-pressuring caused by
hydrocarbon generation as well as by compaction. Once released into porous and
permeable carrier beds or into faults planes, oil and gas then move upwards towards the surface
in an overall buoyancy-driven process known as secondary migration.
 References

1. Hyne N.J. (2001). Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and
Production. PennWell Books. p. 164. ISBN 9780878148233.
2. Gerard Demaison: "The Generative Basin Concept" in: American Association of
Petroleum Geologists (AAPG) Memoir #35 : "Petroleum Geochemistry and Basin
Evaluation", 1984 , Edited by Gerard Demaison and Roelof J. Murris, ISBN 0-89181-
312-8
3. Kimmeridgian Shales Total Petroleum System of the North Sea Graben Province –
USGS Bulletin
4. ames, K.H. 2000. The Venezuelan hydrocarbon habitat, Part 2: hydrocarbon occurrences
and generated-accumulated volumes. Journal of Petroleum Geology, 23, 133–164
5. Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results
Summary – USGS Bulletin
6. Total Petroleum Systems of the Paleozoic and Jurassic, Greater Ghawar Uplift and
Adjoining Provinces of Central Saudi Arabia and Northern Arabian-Persian Gulf –
USGS Bulletin