--> Abstract: Paleozoic Gas Charging in the Ahnet-Timimoun Basin, Algeria, by S. J. Cawley, N. P. Wilson, T. Primmer, N. Oxtoby, and B. Khatir; #90956 (1995).

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Abstract: Paleozoic Gas Charging in the Ahnet-Timimoun Basin, Algeria

Stephen J. Cawley, N. P. Wilson, T. Primmer, N. Oxtoby, B. Khatir

The Ahnet-Timimoun basin, Southern Algeria, contains significant gas reserves expelled from originally oil prone Silurian and Frasnian shales. The gas is reservoired inn Devonian and Carboniferous clastics in inversion anticlines formed, primarily, during the Hercynian orogeny. Integration of organic and inorganic geochemical techniques, such as AFTA, ZFTA, fluid inclusion analysis, vitrinite and chitinizoan reflectance, is entirely consistent with gas generation 300 +/- 30MY, immediately prior to or synchronous with the Hercynian orogeny. Data from gas fields has shown the remobilisation of gas during post Hercynian tectonics.

A "two-event" heating/cooling history is proposed:-

1. Maximum burial and palaeotemperature at ca. 300 +/- 30MY prior to or synchronous with Hercynian uplift and cooling.

2. Cooling from a secondary peak (lower than maximum) palaeotemperature at ca. 30-60MY following Cretaceous burial.

Calibrated thermal modelling indicates that Palaeozoic source rocks were-heated above 200 degrees C in the Late Carboniferous. Such high temperatures are consistent with the widespread occurrence of pyrophyllite in Silurian shales.

Two end-member thermal models can account for the observed maturities. The first is a constant high Pre-Hercynian heat flow which rapidly decreases during Hercynian uplift to remain at Present Day values of 50-75mW/m2. Gas expulsion in this case commences much earlier than trap formation. The second is "normal" heat flow of ca. 50mW/m2 until ca. 310My with a rapid increase at ca. 290My followed by an equally rapid drop to constant present day values - in this model, petroleum generation and expulsion is late in relation to structuring.

Burial history analysis, integrated with fluid inclusion and minus cement porosity analyses on associated reservoir sandstones are only consistent with the latter model, ie., a major transient heat pulse of unknown, but most likely, short duration associated with the Hercynian orogeny. This permitted gas to be trapped during synchronous structuring, whereas oil was lost from the system before trap formation. Thus the preferred geochemical model is for major gas generation and entrapment during aversion tectonics.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France