Source Rocks
Source rocks are fine grained shales or carbonate rocks containing a certain amount of organic matter.
Source Rock -- Shale
Experience have shown that a shale must contain at least 5% organic matter or 0.5 % total organic carbon to be able to generate sufficient amount of petroleum to be classified as a source rock.
Kerogen Type -- Ⅲ
It is normally derived from higher landplants that often have been exposed to oxidation on land before deposition. Plant remains can often be identified in type Ⅲ kerogen. The potential for generating oil is very low, but it may have a good potential for generating gas.
Evaluating Quality Using Rock-Eval HI/OI
Type Ⅰ has the highest hydrogen content; type Ⅲ, the lowest. To determine the kerogen type present in a source rock, plot the hydrogen and oxygen indices on a modified Van Krevlen diagram.
Kerogen Type -- Ⅰ
It is formed by accumulation of algal matter, dominantly from fresh water environments. This type of kerogen has a good potential for generating oil and gas.
Trap Evaluation
Accumulation of petroleum in a trap = Source rock × Maturation × Migration × Reservoir rock × Trap (including seal) × Timing
Evaluating Source Rock Richness-TOC Generation Potential Poor Fair Good Very Good Wt % TOC, Shales 0.0 - 0.5 0.5 - 1.0 1.0 - 2.0 2.0 - 5.0 Wt % TOC, Carbonates 0.0 - 0.2 0.2 - 0.5 0.5 - 1.0 1.0 - 2.0
Evaluating Source Rock Richness-Pyrolysis
During the analysis, the hydrocarbons already present in the sample are volatized at a moderate temperatures.
Trap Evaluation
Trap Evaluation
A trap consists of a geometric arrangements of permeable (reservoir) and less-permeable (seal) rocks which, when combined with the physical and chemical properties of subsurface fluids, can allow hydrocarbon to accumulate.
Kerogen Type -- Ⅱ
Type Ⅱ kerogen is formed by the accumulation of organic matter from phytoplancton, zooplancton, and bacteria in reducing conditions in marine environment. It has a good potential for generating oil. The majority of the large oilfields in the world have source rocks with type Ⅱ kerogen.
Trap evaluation is to estimate the probability of discovery prior to drilling of a mapped prospect.
Trap Evaluation
Accumulation of petroleum in a trap = Source rock × Maturation × Migration × Reservoir rock × Trap (including seal) × Timing
TOC % =[0.082(S1+S2)+S4]/10.
Evaluating Source Rock Richness-Pyrolysis
Evaluating Source Rock Quality Type of Organic Matter
Oil is mainly generated from organic matter derived from microorganism living in aquatic environments, while organic matter that comes from terrestrial plants mainly generate gas.
Kerogen is the most important part because by thermal alteration it will convert to petroleum.
Molecular Structure of Kerogen
Kerogen Type -- Ⅰ
Type Ⅰkerogen have a high H/C (>1.5) and low O/C ratio. It consists mainly of aliphatic chains and the number of nuclei is low.
Source Rock -- Carbonate
For a yet unknown reason, 3% organic matter or 0.3% total organic carbon is enough for a carbonate rock to be classified as a source rock.
Kerogen Type -- Ⅲ
Type Ⅲ kerogen has low H/C (0.7--1.0) and a high O/C ratio. It contains a large number of nuclei and only minor amounts of aliphatic chains.
The amount of hydrocarbons are measured and recorded as a peak known as S1.
Evaluating Source Rock Richness-Pyrolysis
Next pyrolyzed is the kerogen present in the sample, which generates hydrocarbons and hydrocarbon-like compounds (recorded as the S2 peak), CO2, and water.
Kerogen
The transformed organic matter consists of a mixture of complex organic compounds. A small part can be extracted by organic solvents and is called bitumen. The rest, which is the major part, is not extractable in organic solvents and is called kerogen.
Kerogen Type
Kerogen Quality
Kerogen Type Predominant Hydrocarbon Potential Amount of Hydrogen Typical Depositional Environment
Ⅰ
Oil prone
Abundant
Lacustrine
Ⅱ
Ⅲ Ⅳ
Oil and gas prone
Gas prone Neither (primarily composed of vitrinite or inert material)
Moderate
Small None
Marine
Terrestrial Terrestrial(?)
Evaluating Quality Using Rock-Eval HI/OI
HI = S2 (mg/g)/%TOC×100
OI = S3 (mg/g)/%TOC×100
Evaluating Quality Using Rock-Eval HI/OI