Latest News on Marine Geology: Dec 2020

The role of downhole measurements in marine geology and geophysics

During the last 25 years, downhole measurements have been increasingly used for scientific applications in marine geology and geophysics, particularly in deep‐sea drilling operations. Used mostly by the oil industry to map promising formations for exploration and production of hydrocarbons, a variety of instruments have been developed that can be lowered down drill holes to extract information about the subsurface geology. In the last decade, advances in computers, software, and data transmission have greatly increased the amount and quality of data that such instruments can provide. Relatively new instruments that image the borehole wall with high resolution can reveal layers and faults that previously could be seen only in core sections. Downhole measurements play a crucial role in linking core data with regional geophysical surveys and in providing data where core sections could not be obtained. Examples of recent scientific applications and approaches are presented that address previous problems with data quality and changes in properties over time after a hole is drilled. The role of downhole measurements is discussed for two broad areas of research: the structure and composition of the Earth’s crust, most of which is formed at mid‐ocean ridges, and past changes in Earth’s environment recorded in the deep‐sea sediments overlying the crust. Finally, new emerging technologies and experiments that promise significant advantages over current methods for downhole measurements in marine geology and geophysics are discussed. [1]

Bathymetry and Marine Geology of the Andaman Basin, and Tectonic Implications for Southeast Asia

The Andaman Basin extends 1200 km from Burma to Sumatra and 650 km from the Malay Peninsula to the Andaman and Nicobar islands. The eastern portion of the basin is the Malay continental margin, a 250 km wide shelf which was cut into Malayan Paleozoic and Mesozoic rocks during the Early and Middle Tertiary. Subsequent folding and faulting modified the shelf, forming at least two major terraces, the Sumatra Shelf-basin and a 650 km long continental slope. Only a discontinuous veneer of Cenozoic sediments covers the continental margin, except for the Sumatra Shelf-basin, in which over 1 km of sediment has collected.

The Andaman-Nicobar Ridge, western boundary of the basin, consists of an Upper Cretaceous serpentinite-ophiolite-radiolarite core overlain by Paleocene to Miocene graywackes and shales at least 3000 m thick. Between the ridge and the Malay continental margin lies the Central Andaman Trough, two elongate basaltic seamounts 220 km long, and a complex system of rift valleys and associated smaller volcanic seamounts. Maximum depths of 4400 m in the Andaman Basin are located in the major branch of the rift system, the 700 km long Nicobar Rift Valley which is the proper boundary between the Andaman-Nicobar Ridge and the Central Andaman Trough. [2]

Marine geology of the Rockall Plateau and Trough

The Rockall Plateau is an extensive shallow water area located south of Iceland and west of the British Isles: it is separated from the British Isles by the 3000 m deep Rockall Trough. Rockall Island, composed of 52 ± 9 Ma aegirine-granite is the sole subaerial expression. The Rockall Plateau is interpreted as a continental fragment or microcontinent isolated during the sea floor spreading evolution of the North Atlantic Ocean. A geological reconnaissance of the Rockall Plateau and Trough has been made by using a 650 cm3 (40 in3) seismic reflexion profiling system, supplemented by sparker (8 kJ) profiles on Rockall Bank and arcer (60 kJ) profiles across the margin west of the British Isles. Stratigraphic interpretation of these profiles has been aided by deep sea drilling data, bottom sampling on Rockall Bank and by the relation between the various reflecting horizons and oceanic basement dated by oceanic magnetic anomaly identifications. Analysis of the microtopography of the area has given information on Post-Palaeogene sedimentation processes. Three major sedimentary basins are present in the area. The Hatton-Rockall Basin is developed in thinned continental crust on Rockall Plateau. The Rockall Trough is developed on continental crust and includes oceanic crust believed to have been generated in Late Jurassic-Early Cretaceous time. The Porcupine Seabight may be developed on thinned continental crust. All three basins have a faulted basement and exhibit a history of progressive and/or intermittent subsidence. The subsidence phases correlate closely with estimated changes in sea-floor spreading rate. This correlation and the regional pattern of uplift and subsidence is discussed with reference to the effects of thermal subsidence and differential loading of the lithosphere beneath continental margins. Post Upper Eocene sedimentation throughout the area was characterized initially by widespread chert deposition and subsequently by differential deposition of Early Miocene to Recent oozes. The onset of widespread differential deposition in the Early Miocene indicates the present near bottom-circulation was established at this time and may be related to subsidence of the Iceland-Faeroes Ridge. The relation between differential deposition, topography and circulation is discussed in terms of flow around obstacles. [3]

Comparative Evaluation of Rheological Properties of Standard Commercial Bentonite and a Locally Beneficiated Bentonitic Clay from a Marine Deposit in Upper Benue Basin, Nigeria

Nigeria is ranked the seventh largest exporter of crude oil in the world. On a daily basis, it produces over two million barrels of crude Oil from hundreds of Oil wells drilled in the Niger Delta area. Not a single of these Oil wells are drilled without the use of Bentonite. Bentonite is a clay powder that is mixed with water (drilling mud) and forced through a drilling string into the wellbore during drilling operations in order to float and lift drilled cuttings out of the well, control the down-hole temperature, lubricate drilling bit, prevent corrosion and stabilize the wall of the hole from collapsing among other things. A very large sum of money is being spent by the Oil companies operating in Nigeria on the importation of millions of tons of Bentonite from overseas for Oil well drilling. Nigeria has large reserve of Bentonitic clay deposit that if properly utilized will go a long way in supporting the Oil well drilling demand in the country. This will translate into retaining the large sum of money sent overseas for importation of foreign Bentonite, create employment opportunities, bring external investment and boost the country’s economy. Though the Bentonite deposits studied are Ca-based, it can be beneficiated to a level that is comparable with the standard compliance. This study has beneficiated local Bentonitic clay using Na2CO3 as the beneficiating agent and ion exchange as the procedure. The rheological properties of the beneficiated clay were determined together with that of a standard commercial Bentonite and compared with the United State API grade and European OCMA standard for complience. The recorded values for viscosity at 600rpm, yield, filtration, fineness and moisture content of the locally beneficiated Bentonitic clay and the standard commercial Bentonite are 35, 1 -6, <15, 2.8%, 8% and 27, 2.3, <15, 3.7%, 12%, respectively. While these values for the United State API grade and the European OCMA standards are ≥30,≤3, ≤15, ≤4%, ≤13% and ≥30, ≤6, ≤16, ≤2.5%, ≤13%, respectively. This indicates that the local Bentonite reserve can be beneficiated using Na2CO3 as beneficiating agent and ion exchange as beneficiating procedure to a level that is even a little better than the studied standard commercial Bentonite also studied. [4]

Geological, Geophysical and Seismological Investigations for Siting of Seismic Stations in Minna and Abakaliki, Nigeria for Data Reliability

The goal of this study is to adopt international standard criteria to carefully select sites to host seismic stations in Minna and Abakaliki, Nigeria, that can give reliable seismic data with high network detectability, monitor local seismicity and record local events with improved accuracy. By this, mistakes made during the construction of existing stations in Nigeria would be avoided systematically. To achieve these therefore, detailed field assessments were carried out to select most suitable site amongst the four potentials sites earlier proposed to host the sensitive seismic equipment for each area. These steps were later followed with detailed geological, geophysical and seismological investigations to ascertain the rock types at the sites; determine the depth of the bedrock where the sensor would be placed; and evaluate the signal to noise ratios and sources of possible noise to the pre-selected sites. Results from the various investigations have helped a team of seismologists, geologists, geophysicists and GIS experts drawn from the Centre for Geodesy and Geodynamics (CGG), Federal Ministry of Science and Technology and the National Emergency Management Agency (NEMA), to select most reliable sites for seismic stations in Minna and Abakaliki. As this is the first time these kind of investigations have been undertaken to select a site for seismic equipment in Nigeria, it is expected this study would set standard practices for future site selection for sensitive equipment in Nigeria and in the sub-region. [5]

Reference

[1] Goldberg, D., 1997. The role of downhole measurements in marine geology and geophysics. Reviews of Geophysics, 35(3), pp.315-342.

[2] Rodolfo, K.S., 1969. Bathymetry and marine geology of the Andaman Basin, and tectonic implications for Southeast Asia. Geological Society of America Bulletin, 80(7), pp.1203-1230.

[3] Roberts, D.G., 1975. Marine geology of the Rockall Plateau and Trough. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 278(1285), pp.447-509.

[4] Abdullahi, A., Ibrahim, A. A., Muhammad, M. A., Kwaya, M. Y. and Mustapha, S. (2011) “Comparative Evaluation of Rheological Properties of Standard Commercial Bentonite and a Locally Beneficiated Bentonitic Clay from a Marine Deposit in Upper Benue Basin, Nigeria”, Current Journal of Applied Science and Technology, 1(4), pp. 211-221. doi: 10.9734/BJAST/2011/445.

[5] Afegbua, U. K., Yakubu, T. A., Tsalha, M. S., Ologun, C. O., Oluwadare, T. S., Duncan, D., J. Uwazuruonye, J. U., Okwunwa, G., Ekweuzoh, P., Enang, M. and Agoro, O. A. (2017) “Geological, Geophysical and Seismological Investigations for Siting of Seismic Stations in Minna and Abakaliki, Nigeria for Data Reliability”, Current Journal of Applied Science and Technology, 19(1), pp. 1-22. doi: 10.9734/BJAST/2017/31078.

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