ABSTRACT

In Tunisia, most borehole temperatures used to constrain the thermal histories of sedimentary basins were previously corrected using various methods, set to best-fit domains other than the Tunisian basins. This study aimed to propose a new method of borehole temperature correction suitable for the Jeffara Basin, southeastern Tunisia. A total of 92 temperature values including bottomhole temperatures (BHTs) and drill stem test (DST) temperatures were collected from 11 boreholes in the area. The correction method consists of plotting BHT versus DST. The derived equation BHTcorrected=1.036BHTuncorrected+5.9 generates corrected temperatures close to formation temperatures.

An error temperature curve following the equation y=2×107x2+7.57 was calculated (x = depth in meters and y = temperature error in °C). The error varies from +7.4° to +9.4°C (+45.32° to +48.92°F) at depths below −274 m (−898 ft). A temperature error (ΔT°) is proposed according to depth, and to the time elapsed since mud circulation ceased intervals.

To test the method consistency, the Silurian Tannezuft hot shales Formation thermal history is simulated using the BasinMod 1-D™ software program. Two runs were tested based on the new method: (1) noncorrected BHT and (2) corrected BHT.

The first run indicates that the Silurian source rock is at an early stage of the oil window. The second run shows that the source rock is mature and even at a late mature stage toward the north. Oil expulsion has occurred even at an oil saturation expulsion threshold of 5%. The results explain gas condensate production in the study area.

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