A Simple Geotracer Compositional Correlation Analysis Reveals Oil Charge and Migration Pathways

Abstract

A novel approach, based on geotracer compositional correlation analysis is reported, which reveals the oil charge sequence and migration pathways for five oil fields in Saudi Arabia. The geotracers utilised are carbazoles, a family of neutral pyrrolic nitrogen compounds known to occur naturally in crude oils. The approach is based on the concept that closely related fields, with respect to filling sequence, will show a higher carbazole compositional correlation, than those fields that are less related. That is, carbazole compositional correlation coefficients can quantify the charge and filling relationships among different fields. Consequently, oil migration pathways can be defined based on the established filling relationships. The compositional correlation coefficients of isomers of C1 and C2 carbazoles, and benzo[a]carbazole for all different combination pairs of the five fields were found to vary extremely widely (0.28 to 0.94). A wide range of compositional correlation coefficients allows adequate differentiation of separate filling relationships. Based on the established filling relationships, three distinct migration pathways were inferred, with each apparently being charged from a different part of a common source kitchen. The recognition of these charge and migration pathways will greatly aid the search for new accumulations.

Condensation Mechanism of Hydrocarbon Field Formation

“…It is assumed that oil migration occurs due to buoyancy forces along restricted pathways at first vertically and then by migration below a sealed surface to a trap location. The migration pathways are difficult to determine; thus, they are often inferred based on indirect signs such as molecular markers/indices 16 – 18 , oil-bearing fluid inclusions 19 , or a set of these types of indicators 20 . The pathways are also determined based on the morphology of the sealing surfaces, which enable us to predict the potential localization of hydrocarbon accumulation with a certain probability 21 .…”

Benzonaphthothiophene: Molecular Indicators for Thermal Maturity

“…In order to gain a better insight into the formation mechanism of the BNTs, we compared the distributions of the BNTs and methyldibenzothiophenes (MDBTs). The methyldibenzothiophene ratio (MDR = 4-MDBT/1-MDBT) has been widely used as an effective indicator of maturity. ,− The BNT ratio ([2,1]­BNT/[1,2]­BNT) was widely used in tracking oil migration. ,, We compared the [2,1]­BNT/[1,2]­BNT and 4-MDBT/1-MDBT ratios with the increasing vitrinite reflectance. The 4-MDBT/1-MDBT values gradually increase with the increase of vitrinite reflectance with %Ro > 0.6, which is consistent with the phenomenon reported by the previous literature. ,, However, the 1-MDBT concentration in high mature coals with %Ro > 1.2 are very low, resulting in the MDR values of >30 (Figure a).…”

“…59 As can be seen from Figure 7, BNTR-1 and BNTR-2 values both show an overall increase with the increase of F 2 , MPI1, and %Rc (MPI1) values, which The BNT ratio ([2,1]BNT/[1,2]BNT) was widely used in tracking oil migration. 5,8,19 We compared the [2,1]BNT/ [1,2]BNT and 4-MDBT/1-MDBT ratios with the increasing vitrinite reflectance. The 4-MDBT/1-MDBT values gradually increase with the increase of vitrinite reflectance with %Ro > 0.6, which is consistent with the phenomenon reported by the previous literature.…”

“…Benzonaphthothiophenes are important organosulfur compounds in coal tar, coal liquids, shale oils, sedimentary rocks, and oils. − Much work has been achieved on their identification and occurrence in oils and application in tracking oil migration. − However, little is known about the distribution, controlling factor, and geochemical significance of benzonaphthothiophenes in sedimentary organic matter.…”

“…Panda et al proposed a migration indicator, namely, the BNT ratio ([2,1]­BNT/[1,2]­BNT), and used it to track oil migration in Saudi Arabian fields. Subsequently, the BNT ratio was successfully applied in mapping migration pathways and constructing filling sequences of oil fields, in comparison with benzocarbazoles. , However, the effect of thermal maturity on the distributions of BNTs is not clear, which may cause some problems in the applicability of BNTs to track migration pathways if the thermal history is not well constrained and when dealing with samples of variable thermal maturities.…”