Characterisation and evaluation of rock aquifer in Jurong formation

Abstract

Geologically complex of Jurong Formation underlies part of Southern Johor Bahru which comprises of well cemented and consolidated volcanic-sedimentary rocks. Unlike alluvial aquifers, groundwater extraction from hard rocks is challenging due to their complex and highly heterogeneous nature. This study aims to characterise and evaluate rock aquifer in Jurong Formation. Methodology adopted in this study includes review of previous boreholes data and analysis of geomorphology and lineaments. During field work, 5 outcrops located near studied area were mapped. The geological profile of study area changes from fresh pyroclastic volcanic rocks to semi-consolidated older alluvium in west-east direction. Sandstone (sedimentary roof pendant) are overlain by members of Jurong Formation (Bukit Resam Clastic Member overlying Gunung Pulai Volcanic Member) at study area. The average strike/dip angle of beddings is 067o/42o. Twelve resistivity and five seismic lines were also surveyed for subsurface evaluation. Resistivity value of <1000 Ohm.m is suggested to be fractured zone. Whereas, depth of bedrock with seismic velocity of >3200 m/s ranges from 20 m to 30 m and deeper (40 m) towards southern region. Upon understanding the subsurface geology, 3 exploratory wells, namely W1-W3 were drilled. A downhole camera survey was carried out to investigate the characteristics of the fracture system. Fracture density in W2 (175/150 m) is higher than W1 (79/165 m). Gently dipped (<20o), single set of joints with tight to open aperture (<0.1 – 4 cm) are dominant in both wells. Major orientation of discontinuities is north-east (0o-90o) to south-west (180o-270o) which is subparallel to bedding plane and lineament orientations. The pumping test indicated that W1 and W2 act as double porosity and unconfined aquifer with yields of 37.85 m3/hr and 60 m3/hr respectively. The groundwater in the study area is fresh and non-saline with hardness of soft to hard. The groundwater transitioned from CaCl (W4) to CaNaHCO3 (W1) as well as from CaHCO3 (W2) to CaNa (W3). Based on the hydrogeological characterization, it is found that the fractures with major opening (>4 cm) and litho-contact found in fresh zone domain at 69-165 m depth dominantly control the groundwater occurrence and productivity at the study area. In addition, the nature of fractures such as density and aperture influence the groundwater productivity. Meanwhile, the primary porosity of rocks is found not to control groundwater productivity except at completely weathered to residual soil zones. Yet, the productivity at weathered layer is insignificant in the study area. The fractured zones at depth <102 m are less productive with airlift yield of <11 m3/hr, characterised by fracture with major opening (>4 cm) less than 2 nos/5 m with transmissivity and hydraulic conductivity of 1.65-2.55 m2/d and 0.01-0.02 m/d respectively. At depth of 110-125 m, joints are characterised by fracture with major opening (>4 cm) less than 4 nos/5 m with airlift yield, transmissivity and hydraulic conductivity of 15 m3/hr, 3-3.3 m2/d and 0.02 m/d respectively. Meanwhile, the most productive fractured zones located at depth of 142-165 m are characterised by 2-4 sets of joints with major opening aperture (>4 cm), gentle dip angle (20o) and density of 3-12 nos/ 5 m along with multiple open fractures (>0.1 cm). Significant increase in airlift yield up to 55 m3/hr indicates capacitive function of the zones with hydraulic conductivity and transmissivity of 0.01-0.12 m/d and 3-17.1 m2/d respectively. Based on the results, a conceptual model on fractured system is proposed for fresh zone domain which controls the rock aquifer productivity.