Petrophysical Evaluation and Permeability Estimation of the Late Cretaceous Sa’di Formation in the Halfaya Oil Field, Southern Iraq

Abstract

This research evaluates the Sa'di Formation within the Halfaya oil field in southern Iraq through an integrated petrophysical analysis of six wells. A full set of wireline logs was interpreted using Techlog software, including Caliper, GR, NPHI, RHOB, resistivity, and sonic, to determine petrophysical properties of the formation, including lithology, Shale Volume, porosity, fluid saturation, and permeability. Cross-plot techniques (Neutron-Density and MID) confirm that the Sa’di reservoir is composed predominantly of limestone (calcite) with minor shale content. Shale volume is generally low (<10%), minimizing its effect on porosity. Porosity logs indicate moderate reservoir quality, with effective porosities averaging 10-20% in the main pay zones. Water saturations were calculated using Archie’s equation, revealing hydrocarbon saturations of 60 –70% in the best intervals. Low permeability was estimated (0.01-10 md) by correlating core measurements with the Flow Zone Indicator (FZI) method, the dataset included the results of 100 core sample analyses from three wells (HF-137, HF-161 and HF-316). yielding a strong agreement between core-derived and log-derived permeability R2 = 0.91. The Sa’di reservoir is subdivided into an upper non-productive unit (Sa’di-A) and a lower reservoir unit (Sa’di-B) which contains three subunits (B1, B2, B3) Petrophysical zonation indicates that Sa’di-A is a tight, marly limestone approximately 50 m thick with negligible oil. In contrast, Sa’di-B, approximately 80 m thick, contains the oil-bearing units, with B2 and B3 units exhibiting the best porosity and hydrocarbon saturation. Ultimately, this integrated approach yields critical insights into the quality and heterogeneity of the Sa'di reservoir, providing essential data for effective reservoir management and future development strategies.