Lam Member - The Source Rock And The Reservoir In

 The Marib-Shabwa-Hajar Baisn.

Introduction

.

Lam Member ( Madbi Formation ) is well distributed in the Marib-Shabwa-Hajar Basin ( Fig-1  ). It was first penetrated by exploration wells drilled by Hunt in Block-18. It is the main source rock in the  and responsible for the generation of light oil and Meem Member(Madbi Formation ) responsible for generation of condensate and gas in the Blocks-18 and 5. The petroleum system was  between Lam Member and Alif Formation. The thin sandstone layers of Lam member were carrying hydrobarbons, which were generated from the shaly source rocks of Lam Member to the near-by

  Listric-Normal faults, which in-turn played the role of hydrocarbon carriers from the source rocks to the Alif Clastics Reservoirs.

Recent exploration drilling results in the Blocks-S1 ( An-Nagyah Field) and S-2 ( Al-Nilam-1 well )(Fig-2 ), were Lam Member is the Source rock and the Reservoir). This new understanding has increased the hydrocarbon potentiality of the Basin and has opened totally a new Petroleum System for the Lam Member. In this case the thickness of the Lam sandstone has increase and this had  lead it to turn from hydrocarbon carrier into a reservoir.

Lam Member ( Madbi Formation ).

Synonymy: Upper Madbi Shales Member (Partly) of coastal outcrop area in SW Hadramawt and Shabwa Provinces.

Age: Early or Lower Tithonian. Fossils- Cupressocites oxycideroids, Stenozarotrilites radiatus.

Lithology: Predominantly calcareous mudstones/ with minor sandstones and carbonates. Mudstones/shales grey-brown grey to greenish and dark grey variably calcareous  to non- calcareous, locally sandy  commonly slightly silty, micaceous, occasionally to commonly pyretic and carbonaceous, the sandstones are principally turbidites and occur as thin beds within the argillaceous sequences, locally carbonaceous,  argillaceous, pyritic, fine to very fine, subangular- subrounded, well sorted; the carbonates are principally grayish , firm to hard, silty, variably argillaceous and pyritic, locally oolitic and carbonaceous, often silty and include rare dolomites.

The sands had been derived from the volcanic and possibly metamorphic basement Paleo-highs outcropping the basin vicinity.

Petrophysical  Properties:

Porosity of Lam sandstones  is in the range of 8%- 21%.

Permeability is about 26md to 250md.

Petroleum System

:

In Blocks 18 ( Hunt) and 5 (Jannah-Hunt), the Petroleum System is between Alif sandstones as the main reservoir and Lam source rock. The thin sandstone layers were transferring hydrocarbons generated from Lam shales and bringing them to the near by

faults, which in turn were carrying the hydrocarbons to the Alif sandstones (Fig-3 )

But in Blocks S-1 and S-2, the picture of  Petroleum System is completely different. Oil which was generated by the Lam shale source rocks directly entered in the Lam sandstones and remained there (Fig-4).  The thickness of the Lam sandstones increased and the porosity became better. That’s why it didn’t  enrich neither LISC nor UISC in the An-Nagyah Field and mostly the same Phenomena occurred in the Nilam Prospect

.

Conclusion.

1-Lam sandstones were derived most probably from the western vicinity of basin in Block S-1 but in BlockS-2 they were derived from the  Northern part of the Central Ayad High.

2- In Blocks 18 and 5 have a petroleum System but in Blocks S-1 and S-2 the hydrocarbon accumulation in the Lam sandstone is in- situ (Direct hydrocarbon migration)

References.

1-      Abdul Sattar O. Nani. Relation between Halokinesis and Hydrocarbon Accumulation in Sabatayn and possible Tihamah Basins of Yemen. 2^nd  International Yemen Oil & Gas Conference. 24-25 June-2002. Sharaton Hotel, Sana’a Yemen.

2-      Richard J. Wharton, Mark Sturgess, Ian D. Maycock-Hunt Oil Co. First Joint AAPG/AMGP Research Conference. “Geological Aspects of Petroleum Conference”. October 2-6,1994. Mexico City, Mexcico.

3-      Ziad R. Beydoun, Mustafa A.L. As-Saruri, Hamed El-Nakhal, Ismail N. Al-Ganad, Rasheed S. Baraba, Abdul Sattar O. Nani and Mohammad H. Al-Aawah. International Lexicon Of Stratigraphy, Volume III. IUGS Publication No.34- 1998.