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# Geology/Understanding closure from maps

Question
Sir, I would like to understand how I can read depth maps to say if there any structural closures, dip closure etc. and what is a dip closure, four-way or two-way dip closure?
thank you

Sheeba,

A Geologist must be able to visualize in three dimensions to understand a structure.

The depth maps are the contour maps, which show elevation with reference to a datum, either the Seismic Reference Datum (SRD) or mostly the Mean Sea Level (MSL). The structural or a four way dip closure is same, and the shape of the contours is concentric and mostly ovalish with the contour values increasing outwards on a map. On the contrary the contour values increase inwards in case of a syncline. A two way dip closure is rather a three-way dip closure; it is like a half dome cut by a sealing fault at the upper end. The concentric contours in both  cases (four-way, or three-way) will show increasing values outwards on a map, and downwards (down-dip) in a section.

Suggest make two sketches with concentric contours. On the first put increasing values outwards and on the other, in the reverse order. Make a section along lines across both the sketches. The section will be like a dome for the one having increasing values outwards, and the other one will be of concave shape. Better still take a bowl, put it upside down on a table and draw lines of equal heights all around (with reference to the surface of the table). Hold a horizontal bar above the bowl to simulate a reference datum. You will note that the vertical distance from the bar to the contours increases for every subsequent outer contour (an anticline with four way dip or the structural closure). However, the increase in the value will be in the reverse order when the bowl is placed normally (a syncline).

For the three-way dip closure you simply put your curved left hand against the straight/flat palm of your right hand (a fault), under a table; the table becomes a reference datum. The shape of the semi-circular concentric lines, if drawn on the left hand, will again have increasing value downwards.

Hope it helps.

#### Nusrat Kamal Siddiqui (Mr)

##### Expertise

I am a petroleum exploration geologist with about 40+ years of diversified experience. I can answer questions on stratigraphy, log interpretation, field studies based on set of logs and core results, general geology etc. Can provide guidance for career planning in earth sciences. Can not help in questions about hard rocks, igneous petrology & mineralogy, sequence stratigraphy and seismic.

##### Experience

Petroleum Exploration - field geology, stratigraphy, prospect generation, interpretation of satellite imagery (visual), log interpretation.

Organizations
Retired in May, 2006 as Senior Manager Exploration from Pakistan Petroleum Limited after serving 26 years. Since then continuing in active service with petroleum exploration companies.A cumulative experience of about 40 years covers 30 years in petroleum exploration, 5 years hydrogeology experience in Libya, and about 5 years of dam geology in Pakistan.

Publications
-AAPG Bulletin (Vol.88, July, 2004); paper on Sui Main Limestone (Eocene), a prolific gas reservoir in Pakistan. -Have presented papers on diversified topics ranging from spirituality, flood control, remote sensing and off course geology in France, Nepal, Bangladesh, Sri Lanka, UAE, Kashmir, India and Pakistan that have been published in respective Special Proceedings. -ONGC Bulletin(India) Vol. 44, Number 2, December, 2009; Chapter 19.A & 19.B on basin architecture and stratigraphy of Pakistan, p.402-474.

Education/Credentials
BSc Honours in Geology and Master of Science in Petroleum and Structural Geology. PGD from ITC, The Netherlands.

Awards and Honors
-Remained Active Member of AAPG from 1990-2006; founding member of PAPG (a local affiliate of AAPG). -Life member of SEGMITE & AGID, and Senior Editor of SEGMITE magazine. -Member of the Board of Studies, i. Department of Petroleum & Gas Engineering (BUITMS, Quetta), and ii. Geology Department, Sindh University, Jamshoro. -Member of Stratigraphic Committee of Pakistan. -Represented Mining Journal, UK in Pakistan till 2007. -Chairman of Annual Technical Conference (ATC)& Oil Show, an SPE/PAPG annual event, in 2003. -Chairman Technical Committee (ATC, 2002).