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Paper Number: 198
Porosity
Evolution of Lacustrine Organic-matter-rich Shales in China
ZHAI, X.F. and WU, S.T.
Research Institute of Petroleum Exploration and Development,
PetroChina, No. 20 Xueyuan Rd., Beijing,
zhaixiufen@petrochina.com.cn
___________________________________________________________________________
The porosity evolution in shales is more complex than that in
sandstones, due to special hydrocarbon (HC) generation in shales. Lots
of scientists are trying to interpret this process via SEM observation
on geological samples with different maturities, which is view-based and
can provide useful information. However, it is too difficult to find the
same organic-matter (OM) from different samples. Artificial-maturation
is a good way to get relatively homogeneous samples, and can provide
useful in-situ information to understand changes at different maturity.
However, the current studies only use gas adsorption to provide
quantitative analysis on whole pore system. Moreover, they can’t
separate OM pores from in-organic pores. Therefore, a new method that
can provide information of different pores is urgently needed to help to
understand the porosity changes in shales.
In this study, we use the physical modeling under high temperature
& pressure and X-ray nano-CT scanning data to analyze the porosity
evolution with increasing maturity. Other data including nitrogen
adsorption, XRD, TOC, Rock-eval and SEM observation on the same position
before and after physical modeling help to find the in-situ changes with
increasing maturity. The pressure in physical modeling is 80MPa and the
temperature includes 300 centigrade, 350 centigrade, 450 centigrade and
550 centigrade. The sample is the low mature Chang 7 shale with type II
kerogen from the Ordos basin. TOC=2.23%, Ro=0.67%, and S1=5.1mg/g
The preliminary results are as follows:
(1) Porosity evolution is positively related to maturity according to
the porosity data at different evolution stages. More and more
nano-pores are developed in organic-rich shale with the maturity
increasing. The porosity which is calculated from nano-CT scanning model
increased from 0.56% to 2.06% when temperature increased from 20 degree
C to 550 degree C.
(2) First, porosity decreases rapidly from immature to low mature
stage because of weak kerogen creaking and strong compaction. Then,
porosity increases rapidly when the maturity increasing from low mature
stage to oil window & gas window and abundant organic matter cracks
to generate hydrocarbon (HC).Then, porosity system keeps stable when the
shale enters into over-mature stage and the intensity of both HC
generation and compaction decreases.
(3) The percentage of new-created porosity in shales varies among
organic matter evolution, clay mineral transformation and brittle
mineral dissolution, and the ratio is 6:3:1 respectively. Abundant
organic matter pores occurs when the maturity over 1.2%.