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3-D Carbonate Seismic Stratigraphy of the Berrasian- Valanginian Minagish Field Region, Kuwait

Al-Baghli, Maha *1; Pigott, John M.2
(1) Exploration Studies, Kuwait Oil Company, Ahmadi, Kuwait. (2) University of Oklahoma, Norman, OK.

Much previous exploration in the Minagish Field of Kuwait has concentrated upon prospectivity of the principle structures facilitated by 3D seismic with much less attention directed toward stratigraphic studies. Therefore, questions concerning the seismic imagery of carbonate reservoir quality and seal potential owing to facies changes accompanying allocyclic (global tectono-eustatic) and autocyclic (depositonal) processes with a paragenetic overprint (diagenetic-fracture history) have been only modestly studied. The focus of this investigation is upon the carbonate seismic facies placed within a sequence stratigraphic framework.

In order to characterize the geophysical aspects (facies) of the operational seismic sequences and subdivided parasequences, we employed seismic attributes constrained with the lithostratigraphic information furnished by the boreholes. Those practical seismic attributes which were especially well-suited to the facies analysis of these carbonates are in the accompanying table:

An integrated Galloway borehole motif -Vail seismic sequence procedure indicates eight distinct operational seismic sequences (parasequence sets) which reveal the dynamic interplay between local autocyclic processes (the advance and foundering of carbonate shoaling platforms) versus regionally correlative allocyclic processes (marl flooding accompanying sea level rise and apparent nondepositonal-discontinuity surfaces with sea level stillstands). For this gently subsiding platform edge of the Gotina Intra-shelf Basin during the Berasian-Valanginian, eight parasequence sets are readily observed. These cycles observed in the third order borehole Galloway motifs are consistent with the seismic Vail interpretations and their vertical accommodation space fill dynamics. Although these cycles do not exactly coincide with the formal stratigraphic nomenclature of the area (e.g. the Middle Minagish), they are easily identified on seismic, correspond to Galloway motifs, and are (numbered from bottom to top) as:

OBSERVED BERRASIAN-VALANGINIAN OPERATIONAL SEISMIC SEQUENCES IN MINAGISH FIELD

4b. Ratawi Shale: Increase in vertical accommodation space (rising sea level) and increase in horizontal accommodation space (transgressive platform flooding) as indicated by increased shaliness.

4a. Ratawi Limestone: No change in vertical accommodation space (stillstand).

3b. Upper Minagish B (UMB): Rising sea level accompanied by transgressive platform flooding as indicated by increased shaliness.

3a. Upper Minagish A (UMA): Initial Rise.

2b. Minagish Oolite C (MOC): Rising sea level accompanied by transgressive platform flooding as indicated by increased shaliness.

2a. Minagish Oolite B (MOB): Stillstand, ooid shoal retrogradation and progradation as indicated especially in digitate platform margins.

1b. Minagish Oolite A (MOA): Rising sea level accompanied by transgressive platform flooding as indicated by increased shaliness.

1a. Lower Minagish: Stillstand, Regressive platform progradation from south to north.

Overall, these four rythmic couplets demonstrate the dynamic interplay between allocyclic and autocyclic carbonate depositional processes of a subsiding carbonate basin margin with changes in relatively high energy grainstone-packstone progradation units capped by lower energy wackestone-mudstone flooding units. It is the underlying high energy couplets which exhibit the greater reservoir quality in this field. Within the parasequence sets, the incorporation of the seismic attributes into facies maps and constrained with the borehole information reveals episodes of carbonate ooid bank development and abandonment which corresponds to spatial differences in reservoir quality.

Further seismic facies work integrated with borehole petrophysics and petrography may provide important insight into the petroleum system evolution of the Minagish Field as well as identify reservoir sweet spots.

Seismic attributes that were best sutied to the carbonates facies analysis

SEISMIC FACIES ATTRIBUTE

FAULTS / FRACTURES

TERMI-NATIONS

SEQ BOUND. /UNCON-FORM.

CONTI- NUITY

MOUNDS/ CHANNELS/CLIN-OFORMS

(# BEDS (HETERO-GEN.)

LITH. CHANGE (SHALINESS)

POROS.

GAS

ENVELOPE (REFL. STREN.)

X

X

CHAOS

X

X

COHERENCY

X

X

X

COSINE OF PHASE

X

X

X

X

DIP DEVIATION

X

X

INVERSION (IMPEDANCE)

X

X

INSTANTAN-EOUS FREQ.

X

X

X

INSTANTAN-EOUS PHASE

X

X

X

X

X

INSTNATNEOUS QUALITY

X

X

X

VARIANCE

X

X

X

ZERO CROSSINGS

X

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain