Rachmat Formation

Rachmat Fm

Period:

Cretaceous

Age Interval:

Coniacian - Santonian

Province:
Sirte Basin

Type Locality and Naming

Middle unit of the Rakb Gr

Synonym: Rechmat Fm (Ahlbrandt, 2001). Lower part is equivalent to the sandy Maragh Fm in other parts of the Sirte Basin.

References: Selley, 1997; Ahlbrandt, 2001; Abugares, 2007, Abdalla et al., 2026

Lithology and Thickness

The mid-section of the Rakb Gr is equivalent to the Rechmat Fm, which consists of shales interbedded with minor limestones, sandstones and dolomites. The shales are dominantly grey, fissile to slightly blocky, to slightly non-calcareous, and glauconitic and pyritic in many locations. The dolomites are common in the basal part and characterized by tan

to brown, microcrystalline dolomite. Changes laterally into sands (Maragh Fm). The Rachmat Fm has a maximum thickness of up to 600 m in the Agedabia Trough (Abugares, 2007).

“The Rachmat Fm is a significant petroleum source rock for the oil fields located along the crest axis of the basement highs of the Sirt Basin. The Rachmat shale also provides an excellent seal over the Etel Fm.” (Abugares, 2007)

Lithology Pattern:

Shallow-marine marl

Relationships and Distribution

Lower contact

It unconformably overlies the Aragub carbonate, Etel Fm, the Bahi Fm or Maragh Sandstone Fm or rests unconformably on the Amal Formation or granite basement.

Upper contact

It is mostly conformably overlain by the Sirte Shale, depending on its position in the Sirt Basin (Abugares, 2007).

Regional extent

GeoJSON

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Fossils

Foraminifera

Age

“The microfossil assemblage indicates that this formation is Santonian to Coniacian in age.” (Abugares, 2007)

Age Span:

Beginning stage:

Coniacian

Fraction up in beginning stage:

0.00

Beginning date (Ma):

89.39

Ending stage:

Santonian

Fraction up in the ending stage:

1.0

Ending date (Ma):

83.65

Depositional setting

Shallow-marine (Abdalla et al., 2026)

Depositional pattern:

Additional Information

Compiler:

T. S. Ahlbrandt (2001): The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System, U.S. Geological Survey Bulletin 2202–F

R.C. Selley (1997): The Sirte Basin of Libya. In: African Basins. Sedimentary Basins of the World, 3, edited by R.C. Selley (Series Editor: K.J. Hsu), Elsevier Science B.V., Amsterdam, pp. 27-37.

Abugares, Miloud M. (2007): Deposition, Diagenesis, and Reservoir Development of the Cretaceous Lidam Formation, SE Sirt Basin, Libya. Durham e-Theses. http://etheses.dur.ac.uk/2599/

Elkanouni, I. E. D. (2014). Sedimentology, Sequence stratigraphy and Spatial-temporal Patterns of the late Paleocene Succession, western Sirt Basin, Libya, Durham e Theses. http://etheses.dur.ac.uk/10538/

Abdalla, M, Abdelnabi, A., Zubi, H., Qaysi, S. (2026): Depositional architecture and reservoir quality evaluation of the Early Cretaceous El Abd Sandstone Member: An integrated sedimentologic and diagenetic approach, eastern Sirte Basin, Libya. Jour. Sedimentary Environments, 11:41. https://doi.org/10.1007/s43217-026-00311-7.