DOUBAN, ADEL F., and FAHED AL MEDHADI, Kuwait Oil Company (KOC)

Abstract: Sequence Chronostratigraphy and Petroleum System of the Cretaceous Megasequences, Kuwait

Sequence stratigraphic concepts have been applied to identify genetically related strata and their bounding regional unconformities. Lithological and biostratigraphical data of the Cretaceous sequences in Kuwait were integrated with the global cycle chart of Haq et al. (1987) and the geologic time scale of Harland et al. (1990) to develop a precise and consistent chronostratigraphic scheme. Five stratigraphic megasequences and four major breaks of sedimentation were identified.The establishment of the stratigraphic framework enables prediction of the reservoir, source and seal within the individual system tract. The Cretaceous petroleum system in Kuwait contains medium gravity oil mainly from the Burgan, Zubair, and Wara clastic reservoirs in addition to the Minagish and Mauddud carbonate reservoirs.The source richness, maturation and potential yields indicate a Cretaceous Makhul or older Jurassic source rock origin.
 
 

1-Berriasian - Early Valanginian Megasequence

Makhul Sequence

The sequence was deposited during a transgressive system tract (TST) period in a middle shelf environment of Late Jurassic, Late Tithonian - Early Cretaceous, Berriasian age that corresponds to the time of LZB-1.2, 1.3 and 1.4 shelf margin wedge to transgressive system tract (SMW/TST) cycles (145.5-143.5 Ma.).The dark shale at the base part of the sequence was recognised as a major source rock, capable of generating heavy and medium gravity oil. The geochemical analysis indicates the presence of amorphous, type II oil prone organic matter, in early to late maturity.

Minagish Sequence

The sequence was deposited during a transgressive system tract (TST) period in an inner shelf and in parts deep inner to shallow middle shelf environment with three recognised higher order sequences (MN30, MN20 and MN10) of Berriasian age corresponding to the time of LZB-1.4 (HST) and 1.5 cycles (143.5-137 Ma.).The Minagish MN20 sequence (oolitic unit) is the main reservoir in the southwest of Kuwait.

Ratawi Limestone Sequence

The sequence was deposited during a highstand system tract (HST) period in an inner shelf environment of Late Berriasian age that corresponds to the time of LZB-1.6 cycle (137-136.5 Ma.).

Ratawi Shale Sequence

The sequence was deposited during a late highstand system tract (HST) period in a deep inner to shallow middle shelf environment of Early Valanginian age that corresponds to the time of LZB 2.1 cycle (136.5-135 Ma.) and is separated from the overlying Zubair sequence by a type 1 sequence boundary

Valanginian - Hauterivian Unconformity

A major sedimentary break occurred during Mid.Valanginian - Hauterivian time as recognised by the absence of the LZB-2.2, 2.3,2.4 and 2.5 (135-128.5 Ma.) cycles.
 
 

2-Late Hauterivian/Barremian - Aptian Megasequence

Zubair Sequence

The sequence was deposited during a lowstand to transgressive system tract (LST/TST) period in an inner shelf to shallow inner shelf environment with six recognised higher order sequences (Z10, Z20, Z30, Z40, Z50 and Z60) of Barremian age corresponding to the time of LZB-3.1, 3.2, 3.3, 3.4 and 3.5 cycles (128.5-119.5 Ma.).The facies of the Zubair sequence change from a sand/shale section in the northeast to clean sand in the southwest.Very good reservoir potential exists in the north and northwest area due to the excellent porosity, with reduced reservoir potential toward the northeast due to lower sand content.

Shuaiba Sequence

The sequence was deposited during a highstand system tract (HST) period in an inner shelf environment of Aptian age that corresponds to the time of LZB- 4.1 cycle (119.5-115.5 Ma.).The Late Aptian upper clastic unit was deposited during a late highstand system tract (HST) period in a deep inner shelf environment that corresponds to the time of LZB- 4.2 cycle (115.5-111.5 Ma.) and is separated from the overlying Burgan sequence by a type 1 sequence boundary.

Early Albian Unconformity

A major break in sedimentation occurred during the Early Albian time as recognised by the absence of the UZA-1.1 and 1.2 cycles (111.5-105.5 Ma.).
 
 

3-Albian Megasequence

Burgan Sequence

The sequence was deposited during a lowstand to transgressive system tract (LST/TST) period in an inner to shallow inner shelf environment with five recognised higher order sequences (B10, B20, B30, B40 and B50) of Albian age corresponding to the time of UZA- 1.3, 1.4 and 1.5 (LST/TST) cycles (105.5-100.5 Ma.).The thick Burgan B20 and B40 sand sequences are the main reservoir units with high permeability and porosity over most of Kuwait.

Mauddud Sequence

The sequence was deposited during a highstand system tract (HST) period in a shallow inner shelf environment of Late Albian age that corresponds to the time of UZA- 1.5 (HST) cycle (100.5-99.5 Ma.).The sequence has good reservoir potential in north Kuwait, where it has vuggy porosity and gradually pinches out to the south and west.
 
 

4-Late Albian/Cenomanian - Turonian Megasequence

Wara Sequence

The sequence was deposited during a lowstand to transgressive system tract (LST/TST) period in a deep inner shelf environment of Late Albian — Early Cenomanian age that corresponds to the time of UZA- 2.1 (LST/TST) cycle (99.5-97.5 Ma.).The Wara sequence has a good reservoir potential in south and east Kuwait where thick porous sandstone is present, but poor reservoir potential to the northeast where the shaly facies is predominant.

Ahmadi Sequence

The upper unit of the sequence was deposited during a transgressive system tract (TST) period in a middle shelf environment.The lower unit was deposited during a highstand system tract (HST) period in a deep inner to shallow middle shelf. Both units are of Cenomanian age that corresponds to the time of UZA- 2.1 (HST) and 2.2 (SMW/TST) cycles (97.5-96 Ma.). The dominantly shaly formation represents the uppermost cap rock for the Wara and underlying reservoirs.

Rumaila Sequence

The upper unit of the sequence was deposited during a transgressive system tract (TST) period in a deep inner shelf environment.The lower unit was deposited during a highstand system tract (HST) period in a deep inner to middle shelf environment. Both units are of Turonian age, corresponding to the time of UZA- 2.2 (HST) and 2.3 (SMW/TST) cycles (96-94 Ma.).

Mishrif Sequence

The sequence was deposited during a highstand system tract (HST) period in an inner shelf environment of Turonian age that corresponds to the time of UZA- 2.3 (HST) and 2.4? cycles (94-92 Ma.).The sequence is separated from the overlying Mutriba and Khasib sequences by a type 2 sequence boundary.

Turonian Unconformity

A major break in sedimentation occurred during the Late Turonian-Coniacian time as the UZA-2.4, 2.5, 2.6 and 2.7 (92-89 Ma.) cycles were absent due to a major tectonic uplift which affected the Gulf Area during Late Turonian time.
 
 

5-Coniacian/Santonian - Maastrichtian Megasequence

Mutriba & Khasib Sequences

The Mutriba sequence was deposited during a transgressive system tract (TST) period in a shallow inner shelf environment, while the older Khasib sequence onlaps the truncation surface of the older sediments.The Santonian age corresponds to the time of UZA- 3.1?, 3.2 and 3.3 cycles (89-85 Ma.).

Sadi Sequence

The sequence was deposited during a transgressive system tract (TST) period in an inner to outer shelf environment of Late Santonian - Campanian age that corresponds to the time of UZA- 3.4 and 3.5 cycles (85-80 Ma.).

Hartha Sequence

The sequence was deposited during a highstand system tract (HST) period in an inner shelf environment of Campanian age that corresponds to the time of UZA- 4.1, 4.2 and 4.3 cycles (80-71 Ma.).

Tayarat & Qurna Sequences

The sequence was deposited during a highstand system tract (HST) period in a shallow inner shelf environment of Maastrichtian age that corresponds to the time of UZA- 4.4 and 4.5 cycles (71-68 Ma.).The sequence is separated from the overlying Paleogene, Radhuma sequence by a type 2 sequence boundary.

Late Maastrichtian - Early Paleocene Unconformity

A sedimentary break occurred during the Late Maastrichtian - Early Paleocene time as the TA 1.1 and 1.2 (68-60 Ma.) cycles were not recorded.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England