Lignite zone as an indicator to lost circulation belt: a case study of some locations of Anambra State, Southeastern Nigeria

Abstract

Eighteen (18) water boreholes were studied for lost circulation. When locations of the boreholes associated with lost circulation were plotted on the map of Anambra State a lost circulation belt was observed around the River Niger -Onitsha -Oba -Nnewi axis. Lost circulation intervals range between 20-50m and 75-90m depth and were found to be associated with lignite and peaty clay members of the Ogwashi-Asaba Formation and/or the Ameki/Bende Formation. This suggests that natural rather than anthropogenic factors are responsible for the widespread occurrence of lost circulation in the State, although the latter could aggravate the former. Lignite and peaty clay of the Ogwashi-Asaba and Ameki/Bende Formation cause lost circulation because they are woody to earthy in nature and are easily eroded when they occur above the water table. These conditions favour the development of vugs, leading to increase in permeability and porosity, making the formation prone to lost circulation. Pre-drilling geophysical, geological and hydrogeological investigations are recommended, since these would go a long way to identify the presence or absence of lignite and thus help to avert possible lost circulation intervals. @j JASEM

Overpressure Estimation Based on Velocity Data Analysis in The Subduction Zone of Hyuga-nada at Western Nankai Trough, Japan

“…Note that this result was obtained with an average error of 0.017. Overpressure zones indicate restricted vertical and horizontal permeability, possibly by a high density of faults growth, high shale-to-sand ratios, deposition of impermeable sediments, and montmorillonite diagenesis among other factors [13]. Overpressure might be there, is caused by fluid pressure in the rock body or it might also have correlation with temperature at depth which can lead to dehydration that helped the underthrust sediments to retain such high porosities.…”