25th International Meeting on Organic Geochemistry IMOG 2011

P-355
H2S formation and enrichment mechanisms in Puguang gas
field of the Sichuan Basin, China
Tenger Borzijin 1,2 , Wenhui Liu 1,2 , Bo Gao 1 , Zhongning Zhang 3
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,
3 Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou,, Lanzhou, China
(corresponding author:tenger@pepris,com)
The Upper Permian Changxing - Lower Traissic
Feixianguan formations (P2ch-T1f) is one of the most
important habitats for H2S-bearing gas pools in NE
Sichuan Basin, SW China. High H2S concentrations
(10-17%) in the Puguang gas field add great difficulty
for safe production, thus the mechanisms for H2S
formation and enrichment have therefore become an
important issue for our investigation. Thermochemical
Sulfate Reduction (TSR) involving hydrocarbon
cracking and S/Mg-enriched fluids is commonly
accepted mechanism for H2S formation in the studied
carbonate reservoirs. Sufficient reactants, enough
energy supply, adequate accumulation space and a
rigid reduction environment are the main controlling
factors for H2S formation and the enrichment in the
Puguang gas field.
Dissolution of sulfate can provide a rich source of
sulphur. Laboratory simulation experiments show that
significant amounts of H2S are formed under high
temperatures and in the presence of Mg2+, SO4 2solution
and hydrocarbons. The generation rate is
much higher than that for thermochemical reactions
between gypsum and organic matter, and thermal
degradation of sulfur-rich organic matter. This
indicates that TSR is the main mechanism for the
formation of large amounts of H2S under deep burial
and high temperature, and suggest Mg 2+ is a factor
deserving attention. The close correlation between
the sour gas fields and reservoir dolomitization in NE
Sichuan indicates that there is complex
interrelationship between abundant H2S, TSR, Mg 2+
and reservoir dolomitization. The ocurrence of solid
bitumens in sour gas reservoirs in the Puguang field
is a direct evidence for earlier oil migration and
accumulation, thus providing abundant hydrocarbon
source for later reactions with sulfate-rich
(magnesium) formation waters in the P2ch-T1f
reservoirs.
TSR occurs at high temperatures with an initial
reaction temperature of at least 120°C. The P2ch-T1f
reservoirs in the Puguang gas field were buried to a
depth of 4000 m or 120°C in the Middle Jurassic; by
the Late Cretaceous, the burial depth reached 7500 m
or over 150°C. Currently, it is buried to 4500-6000 m
or 120-130°C. Thus, the P2ch-T1f reservoir fluids were
over the activation energy range for TSR production
from the Middle Jurassic.
There are worldwide occurrences of large to extralarge
sour gas fields, such as the Puguang studied
here and Astrakhan gas field of the Near Caspian
Sea. The common feature of these examples is their
occurrence in dolomitized reef flat facies of palaeouplifts
or palaeo-slope settings, accompanied by large
amounts of solid bitumen. During diagenesis, early
dolomitization and organic acid corrosion lead to high
quality reservoir prior to peak hydrocarbon generation
in the source rocks, thus providing favorable storage
space for early accumulation of large amounts of
crude oil.
H2S, a strong reductant with high levels of chemical
activity can be easily consumed under many
conditions during its formation, migration and
accumulation. In order for H2S to migrate, accumulate
and reside with other gases, additional conditions
must be met beside the normal principles in migration,
accumulation and preservation of natural gas. Ironpoor
fluids, timely development of accumulation
space (specifically in carbonate rocks), in-situ or near
accumulation of hydrocarbons are all important
factors for H2S enrichment and preservation. As a
result, many highly sour gas fields discovered in the
world are found distributed in carbonate rock or
evaporitic sequences rather than in clastic rocks.
Therefore, long distance migration and accumulation
increases the chance of H2S consumption, while
reduction reactions and closed systems are the most
important guarantee for H2S enrichment and
preservation in NE Sichuan Basin.
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