论文部分内容阅读
Pyrolysis,chromatographic,and isotopic techniques were applied to study a suite of rock cuttings,crude oils,condensates,and extracted bitumen/oil shows from the Kaikang Trough of the Muglad Basin,aiming at identifying the potential source rock/s,classifying the discovered hydrocarbons,and determining the genetic relationship between the hydrocarbons with the active source rocks units.Two source rock intervals were identified and characterized as highly potential for hydrocarbon generation in the study area.These include the middle part of the Lower Cretaceous Abu Gabra and the top Upper Cretaceous（Baraka-Ghazal formations）.The former consist of a good-very good organic rich shales（TOC=1.0-4.0%,S2>20 mg HC/g TOC）,of mixed kerogen quality dominated primarily by the type-I oil prone organic matter（HI=600 mg/g）associated with few type-III kerogens.This was deposited in a fresh-water oxic to sub-oxic clay-rich lake system,and is currently thermally mature for hydrocarbon generation on the basin peripheries but over mature in the basin centre.The top Upper Cretaceous source displays a wide hydrocarbon generative potential ranging from fair to very good(TOC=（0.5-4.0%）and S2 yield of（2.0-20 mg HC/g TOC）,indicative of type-II oil and gas prone kerogen（HI=450 mg/g）,derived from an anoxic clay-poor eustrine/shallow marine-coastal plain environment,and thermally mature for hydrocarbon generation only in the basin centre.All of the analyzed crude oils and condensates in this study are waxy（Power points>10o）,with light-medium API gravities.They are grouped into two families（A&B）which correlate very well to two organofacies units within the Lower Cretaceous Abu Gabra source rock.The extracted bitumen/oil shows however,shows a positive correlation with the Baraka Formation of the top Upper Cretaceous source unit and is named in this study as family-C oil.Basin scale’s structural analysis reveals three major rifting episodes,in;Early Cretaceous,Late Cretaceous,and Tertiary,respectively.The first rift was the strongest and characterized by the largest extension and most rapid subsidence.Although the second episode showed less rifting intensity,but was associated with several geodynamic activities such as minor folding and strike-slip faulting which created the original hydrocarbon traps in the area.The third episode showed even less subsidence and ended up with the accumulation of a thin sedimentary veneer.A major basin uplift followed by erosional event is evident during this phase,particularly in the eastern and northeastern margins of the basin.This,in association with a strong thermal activity indicated by the emplacement of some igneous intrusions and low-lying basaltic extrusions in the area,have casted a tremendous impact on both the hydrocarbon quality and distribution in the basin.Basin modeling results indicate normal present-day heat flows in the basin（25-55mW/m2）,close to the average range for a continental crust（58mW/m2）.An uplift event that took place at around 13 Ma,followed by a strong erosional event on the top Tendi unconformity（Oligocene-Mid Miocene）,is quite evident in the northeastern margin（Neem-Azraq areas）leading to the removal of around 1,500 m of sedimentary cover,.Significant quantities of hydrocarbons were generated from the Abu Gabra source at three major stages;an earlier at 83.5 Ma（40-60 Mtons/acre ft）that took place in the basin centre,a middle at 66.5 Ma（40 Mtons/acre ft）in the western flank of the basin（i.e.Timsah area）,and a more recent pulse at 47.5 Ma（10-15 Mtons/acre ft）in the eastern flank of the basin（Neem-Azraq areas）.Generation from the Baraka Formation had begun quite recently（around 20 Ma）and is still ongoing.