Dr. Ahmad Ali Abdillah.

       PEPA- Chairman.

       Co-Author.                            

Introduction

Socotra Island was described by Beydoun and Bichan in 1970 during the mapping. The island is divided into two halves by a NE-SW trending normal fault system. To the east , the area is characterized by the Precambrian granite gneiss. The Precambrian is overlain by low-inclined Cretaceous limestone with quartz sandstone at the base, succeeded by Paleocene-Eocene limestone. ( Fig-1 )

The western part of the area contains surface exposed Cretaceous and younger sediments mostly WNW-ESE and some NW-SE trending normal faults into a series of tilted blocks.

In the offshore, south and southeast of the Island, a major NE trending fault marks the southern edge of the Socotra Shelf.

An E-W trending fault-bounded basement ridge is mapped to the southwest of the island. Islands such as Samhah and Abd el Kuri are situated on this basement ridge. The fault pattern to the north and south of the basement ridge is WSW-ESE to NW-SE trending. The structuration is essentially platformic and characterized by normal faulting.

The wells drilled were Samhah-1, Rukh-1 and Phoenix-1.Oil  stains and gas shows were observed in the Qishn Carbonates and Qishn Clastics, also presence of 53m of volcanic rocks in the Upper-Middle Jurassic age in the well Samhah-1

Geochemical results proved presence of source rocks with TOC reaching 2-3%, HI is about 350-537. The Permian and Triassic sections are situated within the oil window.

It is important to note that in the neighboring country –Somali (Berbara ) oil shows and seeps have been noted in the”Nubian”facies of the Tawilah equivalent Yesomma in a Graben structure southeast of Brebera. Freeoil was also recovered from the Wanderer Limestone (28.3 bbl – 35.2 API) and the “Nubian “ ( 4 bbls – 33-35 API ) at the Dagah Shabel-1 well, some 35km SE of Berbera. The oil is considered to have sourced from the Madbi equivalent Oxfordian-Kimmeridgian Gahodleh-Wanderer Daghani sequence.

Excellent to good reservoirs are present in the Permian , Triassic and Qishn Clastics.

Stratigraphy

Stratigraphy of the surface outcrops and Geology of  Socotra territory was studied by Beydoun and Bichan in 1970 and the subsurface was studied by the oil companies in the offshore area by drilling 3 deep wells. (Table-1 ).

Table-1

Detailed Stratigraphy is in ( Fig-2. )

Tectonic Evolution.

*- Evidence indicates that the area had been subject to a complex tectonic history with repeated episodes of uplift and erosion.

*- Utilizing the biostratigraphic data from the penetrated sections of Samhah-1, Rukh-1 and Phoenix-1, seven unconformity surfaces can be identified.

*- The tectonic evolution of the area can be summarized into four main episodes:-

1- Pre Rift.

2-Syn Rift.

3-Post Rift.

4-Gulf of Aden Rifting.

Hydrocarbon Play.

Source Rocks: TOC- Habshiya Fm 0.72 -0.98 %; Mukalla Fm 0.52-71%; Naifa Fm 0.3-0.63%. Some times reaching 2-3%. Ro reaches up to o.72% Below the depth the SR are getting more matured. The Permian and Triassic Fms are in the oil window.

Reservoir Rocks: Murbat Fm (P) Sandstones ; Sudair-Jilh Fms (T) Sandstones ; Shuqra-Naifa Fms (J) Fractured Carbonates  and Qishn Clastics (K).

Seal:  Shuqra, Naifa and Qishn Compact Carbonates.

Traps:  1-Structural -  Horst type.  ( Fig- 3a )

.

             2- Stratigraphic - due to about 7 unconformity surfaces

                 which were identified. ( Fig- 3b ).

        Possible Petroleum System.

It is possible to have a Petroleum Systems as follow:
 

1-a  Sudair Source rocks / Qishn Clastics.  or

  -b  Murbat  /  Shuqra Fractured Limestone.

     2- Jilh Source rocks /  As above

Conclusion.

1- Permian and Triassic sections are situated within the oil 

    window in the well Samhah-1, could be due to the:

a-Number of unconformities between the Lower Cretaceous

   and crystalline  basement.

b-Presence of volcanic rocks during the Upper-Middle Jurassic

   had accelerated the source rock maturation.

2- Presence of oil stains in the Qishn Clastics and Qishn

    Carbonates proves that, generation already occurred,

    migration took place and most probably the faults played

    the role of hydrocarbon carrier.

Recommendation.

1-     It will be good to have a detailed study on the

Hydrocarbon Play of the Soctra Offshore

Territory. (Fig-4 ).

2-     Future exploration wells should penetrate the basement

and find out its hydrocarbon potential.

References.

-A.A.Abdillah., W.A.King., B.R.Mills.,S.Gardiner.,The Masila Fields, Republic of Yemen.

-British Gas. Block-38, Offshore Socotra Technical Committee Meeting. Sana’a, Yemen. September 9^th  1996.

-Beydoun, Z.R., Bamahmoud, R.M.O., and Nani, A.S.., 1993, The Qishn Formation, Yemen: Lithofacies and Hydrocarbon Habitat, Marine and Petroleum Geology, v. 10.p. 364-372.

-Beydoun, Z.R., As-Saruri,M.A.L. El-Nakhal, H. Al-Ganad, I.N, Baraba,R.S.,Nani, A.S.O. and Al-Aswah, M.H., 1998, International Lexicon of Stratigraphy, v.III, Asis, Republic of Yemen, IUGS Pub.No.34, Fascicule 10b2,245 p.

-Nani, A.S.O., Al-Hamdi,K.A.A., Al-Sharif, A.R.A.M.., 2000, Hydrocarbon Play of the Masila Rift Basin, PEPA, MOM, Republic of Yemen., Sana’a,Yemen.

-Haitham, F.M.S., AND Nani, A.S.O., 1990. The Gulf of Aden Rift: Hydrocarbon Potential of the Arabian Sector. J.Petrol.Geol., 13:211-220.

--Beydoun, Z.R., Bamahmoud, R.M.O., and Nani, A.S.., 1993, The Qishn Formation, Yemen: Lithofacies and Hydrocarbon Habitat, Marine and Petroleum Geology, v. 10.p. 364-372.

-Beydoun, Z.R., As-Saruri,M.A.L. El-Nakhal, H. Al-Ganad, I.N, Baraba,R.S.,Nani, A.S.O. and Al-Aswah, M.H., 1998, International Lexicon of Stratigraphy, v.III, Asis, Republic of Yemen, IUGS Pub.No.34, Fascicule 10b2,245 p.

-Nani, A.S.O., Al-Hamdi,K.A.A., Al-Sharif, A.R.A.M.., 2000, Hydrocarbon Play of the Masila Rift Basin, PEPA, MOM, Republic of Yemen., Sana’a,Yemen.

-Haitham, F.M.S., AND Nani, A.S.O., 1990. The Gulf of Aden Rift: Hydrocarbon Potential of the Arabian Sector. J.Petrol.Geol., 13:211-220.

- Siebens Oil and Gas Ltd., 1978. Final well report Samhah-1 and 1A, Off Socotra Island, Gulf of Aden. 22p.