To study the characteristics of crude oil cracking and isotopic evolution in carbonate reservoirs

crude oil samples were collected from Well TK772 in the Ordovician Yingshan Formation

Tahe oilfield

Tarim Basin.We carried out the simulation experiment of oil cracking into gas process under simulated carbonate reservoir conditions by using the semi-open experiment system "hydrocarbon generation and expulsion simulation instrument under the conditions of stratum-porosity and thermo-compression"

and then analyzed the geochemical characteristics of gaseous products.The experiment results showed that oil cracking process in a carbonate reservoir was mainly influenced by temperature

and the occurrence of the TSR was caused by the adding of magnesium sulfate.TSR affected the oil cracking process and the characteristics of its products

resulting in the drop of temperature for the large scale cracking of heavy hydrocarbons

and the generation of large amount of non-hydrocarbon gases in addition to the consuming of large amount of heavy hydrocarbons.The alkane gas carbon isotopic composition is mainly controlled by thermal evolution degree.The carbon isotopes get heavier progressively along with the increasing of temperature

but the alkane gas carbon isotopic sequence partially reverses (δ

13

C

1

<

δ

13

C

2

>

δ

13

C

3

) at certain temperature intervals when sulfates exist.The study shows that TSR not only could alter the chemical composition of natural gas

but also might be an important factor for alkane gas carbon isotopic sequence reversion that is common in the carbonate gas reservoirs with high content of sulfur.