reasons for the decreasing of api gravity in bangestan crude oil reservoirs in mansouri oil field, sw iran

physical and chemical properties of  crude oil, including viscosity and api gravity, are commonly inherited  from the  early organic matter, environmental conditions in which they were deposited and depth of burial of the source beds. api gravity increases in deeper burial due to higher thermal maturation of organic matter in the source rocks and consequently, generation of lighter saturated hydrocarbons. mansouri oil field in the dezful embayment in northern zagros, sw iran, consists of three successive reservoir horizons including: asmari limestone (oligo-miocence), ilam and sarvak formations (bangestan group) of late and mid cretaceous age. api gravity variations do not follow the international depth order in these horizons, i.e., decrease from 25.83 in the upper reservoir (asmari) to 21.46 in the lower (ilam) and to 25.01 in the lower reservoir (sarvak). geochemical analysis of 5 crude samples from 3 reservoir horizons indicated that frequency distribution of saturated and non-saturated hydrocarbons, as well as polar compounds (resins and asphaltens) in the  asmari and sarvak formations are rather close to each other, but they differ remarkably within the ilam formation, e.g., saturated hydrocarbons maximize to 60.51% in asmari, 50.36% in sarvak, and 37.22% in ilam. however, aromatics decrease to a minimum of 28.64% in asmari and 33.84% in sarvak, but maximize to 41.8% in ilam. on the other hand, crude materials in the ilam reservoir contains 20.98% of polar compounds, while in asmari reservoir; it decreases to a minimum of 10.89%. gas-chromatography analysis of crude samples from the mansouri oiled field-showed that cpi results are rather similar and close to 1 , indicating that oils in the sarvak, ilam and asmari resevoirs are generated from one source rock, which belongs to th  kazhdomi formation. other source potential, i.e., pabdeh formation, was not mature thermally and therefore, did not reach the stage of oil generation. it is concluded that the oil field consists of only one petroleum system. mansouri is an under-saturated oil field and lacks gas cap. consequently, its gas-oil ratio is low and the lowest value is found in the ilam reservoir. minimum frequency distribution of paraffinie hydrocarbon and maximized content of aromatics and polar compounds in this reservoir could be probably due to strong variation of porosity and permeability in the ilam formation.