OTTO, S.C., E. IWANIW, A.C.ALEXANDER and C. DARLINGTON, Robertson Research International Limited, Llandudno, North Wales, UK

Abstract: Post-Cimmerian (Jurassic) Petroleum Systems of the Central Paratethys Region

The end-Triassic Cimmerian Orogeny was the result of the collision of a number of blocks of Gondwanan affinity with the southern margin of Eurasia, and led to the closure of the Palaeo-Tethys Ocean.The effects of this orogenic event are widespread and are recorded in sedimentary basins throughout Central Asia. In basins close to the orogen, pre-Jurassic rocks are deformed and are typically considered to be economic basement; basins lying further from the orogenic front have active Palaeozoic petroleum systems.

In the Central Paratethys area, illustrated in Figure 1, Post-Cimmerian extensional basins contain sequences of continental shales and coals that form source rock units of varying importance, stretching from the Caspian Sea to western China. These source rocks reached initial maturity from mid Cretaceous (Amu-Dar'ya) to Miocene (Tarim) and generated hydrocarbons that migrated into coeval sandstones (e.g. North Ustyurt,Turfan) and overlying Upper Jurassic sandstones (Turgay) and carbonates (Amu-Dar'ya, Afghan-Tajik).Variations in the timing of source rock maturity are dependent on heat flow and subsequent sedimentation; a function of the varying tectonic histories of the area. There is a marked difference in Jurassic tectonics either side of the Talasso-Fergana Fault, being broadly extensional to the west and compressional to the east. Considerable variation also exists in the effects of the end-Jurassic collision of the Lhasa Block and there are significant local and regional differences in uplift, deformation and subsidence histories in the Oligocene-Recent following the collision of the India Block.

Post-Cimmerian Petroleum Systems

Basins in the Central Paratethys area can be divided into a number of groups on the basis of the Mesozoic tectonic regime, the strength of the Tertiary compressional overprint and the thickness of Cretaceous to Tertiary overburden; the influence of these parameters on the presence of both Jurassic source rock intervals and Jurassic-Jurassic petroleum systems can then be analysed (Table 1).

Jurassic rocks are also play elements in other petroleum systems: for example, Jurassic source rocks feed hydrocarbons into younger Cretaceous and Tertiary reservoirs in all basins with mature proven Jurassic source rocks, and Jurassic reservoirs host hydrocarbons generated by both older (Precaspian Basin) and younger (Rioni Basin) source rocks.

It is estimated that Jurassic reservoirs host over 6 billion bbl and over 100 Tcf generated from Jurassic source rocks.

Figure 1. Location map of the basins in the study.

Table 1. Parameters influencing the presence of Jurassic-Jurassic petroleum systems in basins in Central Paratethys.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England