Lithofacies Characterization of the Early – Middle Miocene Succession , Case-Study , South Iraq

The Early – Middle Miocene Ghar and Lower Fars sedimentary succession at the representative oil-well Nu-18 of the Nahr Umr oil field south Iraq; is taken by this study to investigate the sedimentological to reservoir rock facies buildups and related reservoir zonation; as first rock-typing attempt for the both formations. The sedimentological characterization of the Early Miocene Ghar formation is mainly comprised by successive buildups of sands-gravels and sandstones, whereas; the Middle Miocene Lower Fars formation is started by limestone, limestone-marly/marl anhydritic, upgraded into interbedded-series of marl and anhydrite facies, with less-common occurrences of thin-sandstone interlayers, terminated by marl-sandy-section at the upper part of the Formation. A successive depositional succession has established of 2 litho-units for Ghar Formation & 4 members for Lower Fars Formation, with related sedimentary intervals. The composite micro-to-mega pores/multi-fractures system of the carbonate – anhydrite lithofacies buildups, of the Lower Fars Formation reflects a good pore – reservoir characterization framework. Accordingly; the Lower Fars Formation members and the Ghar Formation litho-units has passed into reservoir-layering-scheme, and a reservoir layering system has submitted of: 7 and 2 reservoir-units for Lower Fars and Ghar formations respectively, and symbolized as: LF-B1, LF-B21, LF-B22, LF-B23, LF-C1, LF-D1, LF-D2, and GH-A, GH-B.


Introduction:
The Early-Middle Miocene succession was described and forwarded as a stratigraphic nomenclature represented as a rock unit for the Lower Fars formation of green marl, limestone marly, with anhydrite and sandstone interlayers, identified this formation in south-west Iran [1]. This succession at the well Zubair number-3 Zubair oil field, Basra, south Iraq [2]. In the Iraqi the Middle-Miocene Lower Fars formation stratigraphy as lagoonal to evaporate sequence, composed mainly of anhydrite-gypsum & salt, interbedded with limestone, marl, fine-grained plastics [3] [4].
The Ghar formation comprises mainly of successive-buildups of sands-gravels and sandstones interlayers. The field is around 35Km north of the Basra city as shown in  The main purpose of this work: The aim of this investigation is to construct the first rock-typing system of the studied succession; from stand point of sedimentary litho-facies to related reservoir zonation buildups and accompanied concludes pore-system characterization. The first part covers the sedimentologic succession in the Nahr Umr oil-well Nu-18 via the good rock (core to drill cuttings) availability, the second part covers the reservoir rocks characterization, and to evaluate the rock-potentiality of the both formations in the Nahr Umr oil field.

Discussion: Part One: Sedimentology
The Lower Fars formation top is described in this study as marl greenish-grey, soft/plastic-texture, calcareous, and limestone marly pale-green, plastic to semi-plastic, anhydritic, partly sandy of very-fine/fine-grained-texture, corresponds to (336m) as equivalent-log-depth at this studied oil-well. Generally, it represents lagoonal/evaporate depositional setting. The formation conformably underlies the Upper-Miocene/Pliocene Dibdibba formation, of fluvial to deltaic sands-gravels and pebbles of multi-colors, and sandstone-interlayers. The sedimentary-interval bounded between 336 to 675m represents; marl-sandy, interbedded by marl to anhydrite, and carbonate succession representing the Lower Fars formation.
The Lower Miocene siliciclastic buildups of fluvial/deltaic sands-gravels and pebbles of Ghar formation is encountered from 675 to 780m (total-depth at this well), it is unconformably superimpose the Late Eocene Dammam lagoonal carbonates Formation. Sed-interval 1-1: marl, greenish-grey, sandy, fine-medium grained, partly anhydritic pasty to slightly recrystallized with thin sandstone in-layers of fine-medium grained, vary-color, differently sorted, between 336 to 418m.

The Transition
Part: a depositional-interval from 580m to 612m, (32m) thickness. This sedimentary section is considered a transitional-zone from the lowercarbonate-dominated part to the middle interbedded part of the formation. A serious of carbonate-anhydrite to sandstones-interlayers represents this part.

The Ghar Formation Ghar Sedimentary Units Construction:
This formation originally described by Owen and Naser 1958, at the type-locality Zubair oil-well 3: consists sands & gravels with sandstones, and interlayers of limestones sandy; deposited in fluvial-deltaic-sedimentation all over south Iraq during early Miocene. The Ghar formation was not cored previously and basically studied by drill-cuttings, the top of the formation at the studied-well has identified at 675m; as first appearance of sands and gravels with sandstones calcareous.

The Upper Part: 675 -725m:
It is made-up mainly of sands, gravels & more sandstones mostly of fine/medium to coarse-grained-texture, almost same aforementioned composite-litho/pore system characteristics of the lower part.

Part Two: Reservoir Rocks Characterization
This part has organized to study the rock-facies/pore-system characterization to; delineate reservoir lithofacies layering system. The rock-cuttings and core-depths are corrected to their log-equivalents.

The Lower Fars Formation
According to; the above aforementioned lithological subdivision and composite-lithofacies/pore-system characteristics of this formation with (LF) symbolized items, plates (1) to (6) and Figures (3a & 3b); the following scheme has organized:

Lower Fars Part-A (LF-A):
This part represents the upper-member of the formation and does not maintains any reservoir-rock characteristics due to; non-required petrophysical properties. It is ranked under non-reservoir-rocks and classified as a main cap-rock for the underlying reservoir units.

Lower Fars Part-B (LF-B):
This part of the formation represents the middle-member consists of; many reservoir-
In-parts of peloidal-texture, of micritized-bioclasts and some of micritic-rims, may represent shoal-setting of moderate-energy-action, oily-stained. The facies of microsparitic texture, silty, and shows well-developed micro/meso-vugs oilyimpregnated. It represents intensive-dissolution section with related composite-pore system of well-developed micro-to-mega/solution-enlarged-voids.