AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Integrating Diagenesis and Sequence Stratigraphy for Reservoir Quality Evolution Modeling: Evidence from Fluvial and Shallow Marine Sandstones of the Cambrian-Ordovician Successions in Southwestern Sinai, Egypt
(1) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
(2) Department of Earth Sciences, Durham University, Durham, United Kingdom.
(3) Geological Sciences Department, Kafrelsheikh University, Kafrelsheikh, Egypt.
The distribution of diagenetic alterations and of their impact on reservoir quality was investigated within sequence stratigraphic framework of the Cambrian-Ordovician lower sandstone unit in the southwestern Sinai. Depositional facies and sequence stratigraphy of the intervals were interpreted by field study of the outcrop successions. Detailed petrographic and bulk stable isotope analyses were used to determine the conditions of diagenetic alterations and paragenetic sequences in the sandstones.
The study shows that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within depositional facies and sequence stratigraphic framework. The most abundant eogenetic alterations within the lowstand system tract (LST) and below the sequence boundaries (SB) include dissolution of detrital grains and formation of mechanically infiltrated mud, kaolinite, K-feldspar pseudomorphs and overgrowths. The most common eogenetic alterations within the transgressive system tract (TST) comprise formation of pyrite, glauconite and apatite. Mechanical compaction and formation of pseudomatrix are abundant along the transgressive surface (TS) and parasequence boundaries (PB).
Relative sea level fall and its subsequent sub-aerial exposure and basinward shift of the meteoric water zone results in percolation of meteoric waters into the sandstones in the LST and below the SB and PB that cause extensive dissolution of detrital grains as well as formation of kaolinite and authigenic K-feldspars. During LST, flux of mud-rich surface waters into the fluvial sandstones results in the formation of infiltrated mud. Invasion of marine water into the sandstones as a consequence of relative sea level rise and low sedimentation rates encounter in the TST results in formation of glauconite, apatite and pyrite. Development of pseudomatrix, which forms by mechanical compaction of mud intraclasts, is related to the abundance of mud intraclasts in the lag deposits along TS and PB. Distribution of the mesogenetic alterations including pressure dissolution, formation of quartz overgrowths and dickite is controlled by the types and extent of eogenetic alterations.
This study develops a conceptual model, which constrains the diagenetic evolution pathways in the fluvial and shallow marine sandstones within a sequence stratigraphic context. The model provides insights into the parameters controlling reservoir quality variations.