DIAGENETIC ALTERATIONS IN MIDDLE DEVONIAN SANDSTONES OF NORTHWESTERN LIBYA (WITHIN THE DEPOSITIONAL FACIES AND SEQUENCE STRATIGRAPHY FRAMEWORK). Fadel Abdalla Ibrahim bahr, A.M. Kovkhuto Belarusian Research Geological Exploration Institute. Белорусский научно-исследовательский геологоразведочный институт ул. Купревича, 7, 220141, Минск, Беларусь.

This paper discusses and develops a conceptual model for the spatial and temporal distribution of diagenetic alterations and their impact on the evolution of the reservoir quality of the studied Middle Devonian deposits of the upper part of the Ouan Kasa and Awaynat Wanin formations I and II within the depositional facies and sequence stratigraphy framework.

Based on the sedimentological and sequence stratigraphic analysis of the studied Middle Devonian rocks, two depositional sequences have been distinguished. However, each depositional sequence comprises numerous facies successions that resulted from either single, or amalgamated facies associations related to depositional environments.

The spatial and temporal distribution of eogenetic and mesogenetic alterations that can be constrained within the depositional facies and sequence stratigraphy and have strong impact on the reservoir quality evolution includes eogenetic alterations represented by (i) extensive cementation by the iron oxide in the shoreface highstand system tract (HST) below flooding surfaces and in fluvial incised-valley lowstand system tract (LST) sandstones immediately below the transgressive surface. An extensive cementation by the iron oxide resulted in the porosity deterioration during eodiagenesis; (ii) infiltrated clays in fluvial incised-valley LST sandstones were likely formed during the LST late stage. Infiltrated clays in the shoreface-transition HST sandstones were likely formed via the bioturbation activity at the seafloor; (iii) kaolinite in the fluvial incised-valley LST and shoreface HST sandstones is interpreted to have been formed by the influx of meteoric water during the relative sea level fall and progradation time. The formation of kaolinite during eodiagenesis resulted in the porosity enhancement.

Mesogenetic alterations include the formation of quartz overgrowths in fluvial LST and shoreface HST sandstones. Such a cementation by quartz overgrowths resulted in the porosity deterioration during mesodiagenesis.

Chlorite and illite, which occur in small amounts, are most common in the fluvial LST and shoreface HST sandstones.

This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on the evolution of the reservoir quality of the studied sandstones can be better explained within the depositional facies and sequence stratigraphy framework.