Silicified Microfossils in Nubian (Albian) Sandstones of Sirt Basin, Libya: A Key to the Basin History
详细信息
- 责任者：Zhang Jijun (Exploration Department ; Waha Oil Company ; P.O. Box 395 ; Tripoli ; Libya)
- 刊名：Acta Micropalaeontologica Sinica
- 出版年：2013
- 卷期：30(1)
- 页码：p.29-41
- issnNo：ISSN1000-0674
- isbnNo：CN32-1189/Q
摘要
Over 200 core samples from 2 wells in the Gialo High of the Sirt Basin, Libya, have been conducted for micropaleontological study. A number of silicified marine microfossils are discovered for the first time in the Nubian Sandstones that have been long considered as terrestrial origin prior to Neocomian. Silicified fossils show that in Gialo area the Nubian Sandstones moderately-severely cemented and recrystallized with silica are characterized by subtidal-supertidal deposits with low amplitudes of sea-level fluctuations. The main body of the Nubian Sandstones is dated as Albian using silicified microfossils and nannofossils, although little Aptian deposits might exist in the lowermost part of Well I. Four types of silicified microfossils have been recognized. Type I bearing calcareous cortex, is partially replaced by amorphous silica with distinct original features of fossils, Type II is completely replaced by amorphous silica and has moderately preserved interior and exterior features on smooth surface, Type III is replaced by microcrystalline quartzes with poorly preserved features on surface, and Type IV is replaced by megacrystalline quartzes without marked interior and exterior features. These silicified microfossils are considered to be formed during different stages of diagenesis. Type III and IV (particularly Type IV) are associated with severe cementation of silica in the coarse sandstones. Well primarily developed interstitial conduits between sand grains may allow a large volume of silica-borne fluid flow to travel around, which yields more severe precipitation, cementation and recrystallization of silica than in finely grained deposits. Fossil silicification in the Gialo High appears to be linked to the regional and local tectonic and hydrothermal activities due to frequent occurrences of faulting and volcanisms. Quartz cementation and overgrowths in the Nubian Sandstones during the post-Cretaceous indicated by silicified microfossils have caused great reduction or demise of porosity and permeability, which negatively affect reservoir qua-lity. The discovery of the silicified microfossils in the Gialo opens a new window for biostratigraphic study, paleoenvironmental reconstruction, prediction of porosity and permeability, and basin modeling for exploration success in the Sirt Basin, and possibly the other rifting basins elsewhere in the world.