25th International Meeting on Organic Geochemistry IMOG 2011

P-255
Gas source classification in Paleozoic marine strata, China
Wenhui Liu 1,2
1 Sinopec Research Institute of Petroleum Exploration and Production, Beijing, China, 2 Wuxi Institute of
Petroleum Geology, Sinopec Research Institute of Petroleum Exploration and Production, Wuxi, China
(corresponding author:whliu@pepris.com)
The Paleozoic marine sequences in China are
dominated by carbonate, with subordinate amounts of
clastic rocks. Due to their multiple tectonic history and
deep burial,these rocks has often experienced
multiple episodes of hydrocarbon generation,
expulsion, migration and accumulation. Where the
sedimentary basins are in continuous subsidence,
kerogens in the sediments would undergo continuous
thermal maturation to generate oil and then gas, and
are now in highly mature to over-mature stages with
respect to hydrocarbon generation. Where the
sediments underwent initial burial, then uplift and
subsequent deep burial, the originally thermally
matured kerogens would experience a second-period
(or late) hydrocarbon generation; dispersed
sedimentary organic matter and previously migrated
oils in the carrier beds or reservoirs would also be
subject to thermal cracking to form separate gas
phase,and the dominant hydrocarbon potential in
these situations would be derived from the secondary
cracking process of these paleo oil or bitumen
accumulations. Therefore, natural gases in a given
reservoir are potentially derived from thermal
maturation of kerogens in conventional petroleum
source rocks, thermal cracking of paleo oil
accumulation and / or dispersed bitumens in
sediments, or high temperature hydrogenation of
overmatured sedimentary organic matter. As a result,
it would be prudent and logical to include paleo oil
accumulation, bitumens, whether in concentrated or
dispersed form, in the gas source consideration, in
addition to the conventional hydrocarbon source rocks.
The latter may become the dominant sources for
gases derived in highly mature-over-mature
sedimentary strata.
Traditional scheme to classify hydrocarbon source
organic matter is largely based on the chemical
characteristics of insoluble organic matter, and the
latter were commonly classified into type I, II and III
kerogens. This classification scheme is very useful for
evaluating the biological sources of the sedimentary
organic matter, their thermal behaviors (oil or gas
prone), and general capabilities for oil or gas
generation, under early stages of thermal maturation
that involves only first-order chemical reactions.
However, this scheme is not sufficient to capture the
essence of the prevailing chemical reactions in the
deep subsurface that are responsible for natural gas
generation. To overcome the shortcomings of the
conventional scheme, an alternative classification is
proposed in Fig. 1, where the gas sourcing organic
matter is now subdivided into one of the three general
categories: organic matter that is (1) insoluble in
organic solvent, (2) soluble in organic solvent and (3)
released only after acid digestion of host sediments.
Sedimentary organic matter could occur naturally in
dispersed, concentrated or chemically covalent
bonded form. While the first two forms occur mostly in
traditional hydrocarbon source rocks, coal and oil
shales contain mainly concentrated form of organic
matter. Dispersed soluble organic matter within the
conventional source rocks or carrier beds/reservoirs
can act as the hydrocarbon source for secondary gas
generation. In particular, secondary cracking of paleo
oil accumulation has proven to be one of the most
important sources for many of the gas accumulations
discovered in the Paleozoic marine strata in southern
and western China. Solid bitumens, heavy oils, migrobitumens
and dispersed bitumens in oil source rocks
can be the source for subsequent thermal cracking, to
form light oil or condensate gas accumulations. As
dispersed bitumens in source rocksand carrier beds
are quantitatively very large, they are considered to
be another important source for gas generation.
Results of laboratory thermal simulation indicate that
kerogens in overmatured sedimentary rocks bear
relatively limited hydrocarbon potential, while
substantial amounts of the newly formed gas come
from thermal cracking of dispersed bitumens or preexisting
hydrocarbons. In addition, significant
proportions of polar organic material are incorporated
into rock matrix through organic acid-inorganic
mineral reactions, to form complex or organic salts,
particularly in carbonate rocks. The gas potential of
these organic salts may have been underestimated in
most of the previous studies. Therefore, the interreactions,
inter-conversion and temporal succession
of the different organic matter forms in the
sedimentary strata may be invoked as the primary
reason for the preservation of giant gas
accumulations in ultra-deep carbonate reservoirs
recently discovered in southern China.
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