Introduction

The Clastic succession of Zubair Formations is deposited during the Barremain stage within the Early Cretaceous epoch. This study includes the lithofacies analysis and stratigraphic sequence of the Zubair Formation in four boreholes (Mj-2, Mj-4, Su-2 and Su-7) within the Majnoon and Suba oil fields. The area of study is located in Southern Iraq within the Mesopotamian Zone. The Majnoon oil field is a super-giant oil field located 60 km from Basra Governorate, while the Suba oil field is located about 70 km to the northwest of Basra Governorate (Fig. 1).

Fig. 1
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Location of study area with tectonic subdivisions by (Fouad 2014)

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The Barremain succession represents a part of The Late Tithonian–Early Turonian Megasequence deposited in a large intra-shelf basin contemporaneous with a new phase of ocean floor spreading in the Southern Neo-Tethys. Differential subsidence (and resultant thickness changes) occurred across transverse faults. The axis of the intra-shelf basin shifted towards the eastern Mesopotamian Zone into the Tigris Subzone from its previous position on the Salman Zone and western Mesopotamian Zone (Jassim and Goff 2006).

The progradational Zubair/Ratawi clastic shelf was covered by the Shuaiba Formation carbonates following backstopping of the Zubair and Ratawi Formations (Fig. 2). Jassim and Buday (2006) in Jassim and Goff (2006) suggested that the Zubair Formation depocentre was located at the eastern limit of the Salman Zone, as illustrated by the isopach of the Zubair Formation (Ali and Nassir 1989 in Aqrawi et al. 2010) (Fig. 3).

Fig. 2
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Early cretaceous chronostratigraphic section (Aqrawi et al. 2010)

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Fig. 3
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Isopach map of Zubair Formation After Ali and Nassir, 1989 in (Aqrawi et al. 2010)

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The upper contact of the formation is mostly gradational and conformable with the Shuaiba Formation. While the lower contact is an unconformable surface with Ratawi Formation (Buday1980; Douban and Al-Medhadi, 1999).

The purpose of the present study is a lithofacies distribution and stratigraphic development with knowledge of the tectonic events for this succession during the Early Cretaceous epoch.

Methodology

  • Recover the cores and cuttings for studied wells to prepare thin sections.

  • Examine thin sections to describe the petrography and lithofacies analysis of the studied wells.

  • Study the available well logs (obtained from SOC) and relate the log response to facies changes and stratigraphic settings (Table 1).

    Table 1 Zubair Formation lateral and vertical occurrence
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Lithofacies units of Zubair Formation

The thickness of the Zubair Formation is about 315 m in Majnoon oil field and 450 m in Suba oil field (Table 2).

Table 2 Zubair Formation subdivisions and thickness
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The studied succession is divided into three lithofacies using well logs such as GR, SP, DT and RT. The lower member consists mainly of shale with less sandstone; the middle member is consisting of thick layers of sandstone and the upper member is composed mostly of shale layers.

  • Lower unit

    This zone is characterized by high GR log values with many cycles as aggrading upward (bell shape) and grain size increasing. This unit also has a funnel shape in some positions of studied wells. The thickness of this unit is about 86–115 m in Majnoon oil field and about 60–156 m in Suba oil field.

  • Middle unit

    This zone is represented by low gamma-ray values with many cycles of fine to coarse sandstone (cylindrical shape of GR log) in the upper zone of this unit, and coarse up-ward (hour-glass shape) in some positions. The thickness of this zone is ranging from 125 to 145 m Majnoon oil field and from 140 to 320 m in Suba oil field.

  • Upper unit

    This unit is characterized by high shale volume and the general GR log is almost forming a serrated shape, which refers to relatively increasing upward gamma-ray values. Thickness of the upper unit is approximately from 73m to 75m in the Majnoon oil field and from 84m to 153m in Suba oil field.

Facies Analysis of Zubair Formation

According to the classification of siliciclastic rocks for Folk et al. (1970) and Pettihohn et al. (1973) the Zubair clastic rocks are ranging from quartz arenite to mudstone. In addition to the standard dimensions which used in the descripted of the sedimentary particle size, shape and sorting types after Powers (1953), with response of gamma ray and spontaneous potential well logs to determine the sequence boundaries and paleoenvironment. There are five major lithofacies which are recognized in the studied succession:

  1. 1.

    Quartz arenite well-sorted subrounded lithofacies (I): it represents fine-grained well-sorted sand-dominated rocks with sub-angular to sub-rounded grain shape (Plt.1a). The sandstone in this facies is composed of more than 90% of quartz classified according to Pettijohin et al. (1973) as quartz arenite sandstone, with two modes of grain size: coarse and fine (Plt.1a). This appeared in the middle unit as very low gamma-ray values with box shape of gamma ray and spontaneous potential logs in Suba oil field. While in Majnoon oil field it appeared as three parts within the middle unit.

  2. 2.

    Quartz arenite poorly sorted sub-angular lithofacies (II): it represents a wide range of grain size of sandstone (fine–coarse) and well-rounded to sub-rounded grain shape, within the sand-dominated rocks (Plt.1b). The sandstone in this facies contains more than 90% of quartz to classify as quartz arenite which appeared in the lower part of the lower unit. This is characterized by poorly sorted, very low gamma ray values increasing upward with funnel shape of gamma-ray log.

  3. 3.

    Graywacke poorly sorted lithofacies (III): this lithofacies represents the muddy sand-dominated rocks mainly composed of quartz mineral. It is characterized by poorly sorted graywacke sandstone (Plt.1c), with moderate values of gamma ray (funnel shape). This lithofacies appeared in the upper unit of the Zubair Formation, in addition to muddy parts of the lower unit.

  4. 4.

    Sandy claystone lithofacies (VI): this facies is appeared in the sand lenses within shale layers, characterized by high gamma-ray values with funnel shape. The main component of this facies is mud-dominated rock with quartz grains in angular shape (Plt.1d).

  5. 5.

    Mudstone lithofacies (V): this facies represents the upper unit of Zubair Formation. It is characterized by laminated mudstone (Plt.1e), with high gamma-ray values with bell shape.

Associated facies and depositional environments

Three facies associations (depositional environments) are distinguished in the Zubair Formation: delta plain, backshore and delta front. These are distinguished according to Serra (1987), Emery and Myers (1996) and Malcolm Rider (1999) to determine the minor lithologic change and general trend of grain size by using GR, SP and DT logs with the resistivity log (LLD) response.

Lower unit

The main component of this unit is shale with thin beds and lenses of sandstone divided this unit into two parts: the upper most is sand-dominated and the lower part is consisting of shale-dominated rocks (Fig. 4). This unit is characterized by high gamma-ray and positive SP values with an abundance of aggrading (Box shape) in the lower part and funnel shape in the upper part (Fig. 5). The main lithofacies in this unit are poorly sorted graywacke lithofacies (III) and poorly sorted quartz arenite (II) in the upper part. The lower part is characterized by shale lithofacies (V). This sequence is interpreted to form in a delta plain where the rate of deposition equals the rate of accommodation for the upper part (Fig. 6).

Fig. 4
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Variety of GR and SP log vs. DT and Rt log shapes with lithology for the lower unit of the Zubair Formation (Mj-3)

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Fig. 5
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Variety of GR and SP log vs. DT and Rt log shapes with lithology for the lower unit of the Zubair Formation (Su-2)

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Fig. 6
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Variety of GR and SP log vs. DT and Rt log shapes with lithology for the lower unit of the Zubair Formation (Mj-2)

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Middle unit

This unit is the thicker and the main component of Zubair Formation is sandstone with thin beds of shale observed with two patterns of GR and SP logs, the upper most is bell shaped and the lower part is funnel shaped in the Majnoon oil field (Fig. 6), and serrate shaped in Suba oil field. This unit is characterized by alternating shale and sand beds, where the upper part is with thin layer of sand beds and in the lower part, the sandstone beds are thicker (Fig. 6). The main lithofacies in this unit are quartz arenite well-sorted subrounded lithofacies (I) and poorly sorted quartz arenite (II) in the Suba oil field. While the Majnoon oil field is characterized by sandy claystone lithofacies (VI) and shale lithofacies (V), in addition to limited presence of I and II lithofacies. This sequence represents the deposition in a delta front, where the rate of deposition is more than the rate of accommodation in the Suba oil field. While in the Majnoon oil field, there appeared alternating delta plain association facies with the delta front association facies to refer the increase of the accommodation (Fig. 7).

Fig. 7
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Variety of GR and SP log vs. DT and Rt log shapes with lithology for the lower unit of the Zubair Formation (Su-2)

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Upper unit

The main component of this unit is shale with sandstone inter-layers in addition to high amount of organic matters (hydrocarbon) and pyrite. This unit is characterized by high gamma-ray and positive SP values, and according to the patterns of these logs and the different behavior of the sonic and resistivity logs there are two parts in this unit: the first is fining up-ward (bell shape) in Majnoon oil field, and coarsening up-ward (funnel shape) in Suba oil field (Fig. 8). The upper part of this unit is characterized by abundant of greywacke poorly sorted lithofacies (III), sandy claystone and shale lithofacies (VI, V), while the lower part is sandstone-dominated rocks with a quartz arenite poorly sorted sub-angular lithofacies (I) (Fig. 9). The lower succession is matching with the backshore and marsh-associated facies.

Fig. 8
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Variety of GR and SP log vs. DT and Rt shapes with lithology for the upper unit of the Zubair Formation (Mj-3)

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Fig. 9
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Variety of GR and SP log vs. DT and Rt shapes with lithology for the upper unit of the Zubair Formation (Su-2)

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Conclusions

The studied succession represents the third-order sequence which deposits during the Barremain stage within the Early Cretaceous epoch. During the deposition of the Zubair Formation, the siliciclastic shelf followed a cyclical pattern of evolution from the foreshore depositional mode to delta plain mode. A transgression surface (TS) separates the intratidal shale of Ratawi Formation from the delta associations of the lower unit of Zubair Formation (Figs. 10, 11, 12, 13 and 14).

Fig. 10
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Stratigraphic columnar section of well Su-2

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Fig. 11
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Stratigraphic columnar section of well Su-7

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Fig. 12
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Stratigraphic columnar section of well Mj-2

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Fig. 13
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Stratigraphic columnar section of well Mj-4

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Fig. 14
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Stratigraphic cross section shows the association facies distribution in studied area

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From the lithofacies and electrofacies analyses, and reconstructed paleoenvironments of the Barremain basin in the studied area, there are two stages of the deposition:

The first stage was clear when the facies development was from the delta plain to the delta front association facies within the sand-dominated unit of Zubair Formation (lower and middle units) (Figs. 10, 11, 12, 13 and 14).

This succession is represented by three cycles of transgression system tracts (TST), which were ended with the appearance of the shale lithofacies within the well-sorted quartz arenite lithofacies to mark the maximum flooding surface (mfs) in the Suba oil field (Figs. 10 and 11), while in the Majnoon oil field, these three cycles are represented by alternating delta plain and delta front association with dominated shale lithofacies (Diags. 3 and 4). The deposition continued during still ustand sea level to develop at last to delta front association and start the second stage (Fig. 14).

The second stage started with the appearance of the backshore association facies as a high-stand system tract (HST) within the upper unit. This pattern of deposition (HST) continued until a change in depositional environment from delta front to a backshore in all studied wells. The clear change in the lithofacies was from the sand-dominated (delta front association) high-maturity fine-grained sandstone to shale-dominated lithofacies (backshore) that onsets to a still sea level (Fig. 14). Therefore, the upper part of the upper unit represents the HST and the sea level raised for Mauddud Formation.