The common presence of oil seep
ages in dolostones is widespre
ad in Cret
aceous c
arbon
ate successions of the Kurdist
an Region of Ir
aq. This integr
ated field, petrogr
aphic, chemic
al, st
able C, O
and Sr isotopes,
and fluid inclusion study
aims to link dolomitiz
ation to the origin
and geochemic
al evolution of fluids
and oil migr
ation in the Upper Cret
aceous
Bekhme c
arbon
ates. Flux of hot b
asin
al (hydrotherm
al) brines, which is suggested to h
ave occurred during the Z
agros Orogeny, resulted in dolomitiz
ation
and cement
ation of vugs
and fr
actures by co
arse-cryst
alline s
addle dolomite, equ
ant c
alcite
and
anhydrite. The s
addle dolomite
and host dolostones h
ave simil
ar st
able isotopic composition
and formed prior to oil migr
ation from hot (81&nd
ash;115 °C) b
asin
al N
aCl&nd
ash;MgCl
2&nd
ash;H
2O brines with s
alinities of 18&nd
ash;22 wt.% N
aCl eq. The equ
ant c
alcite cement, which surrounds
and hence postd
ates s
addle dolomite, h
as precipit
ated during oil migr
ation from cooler (60&nd
ash;110 °C) N
aCl&nd
ash;C
aCl
2&nd
ash;H
2O brines (14&nd
ash;18 wt.% N
aCl eq). The yellowish fluorescence color of oil inclusions in the equ
ant c
alcite indic
ates th
at the oil h
ad API gr
avity of 15&nd
ash;25° composition, which is lighter th
an present-d
ay oil in the reservoirs (API of 10&nd
ash;17°). This difference in oil composition is
attributed to oil degr
ad
ation by the flux of meteoric w
ater, which is evidenced by the low δ
13C v
alues (− 8.5‰ to − 3.9‰ VPDB)
as well
as by nil s
alinity
and low temper
ature in fluid inclusions of l
ate column
ar c
alcite cement.
This study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration, degradation and reservoir evolution.