REVEALING AND INTERPRETING OF SUBSURFACE TECTONIC PATTERN OF THE AREA NORTH OF SIWA OASIS, WESTERN DESERT, EGYPT, USING BOUGUER GRAVITY DATA

The studied area lies to the north of Siwa Oasis and to the west of Qattara Depression, western desert, Egypt, between lat. 29o 30` and 30o 30` N` and long. 25o 00 and 26o 30 E` with a total area of about 15600 km2. The Bouguer gravity anomaly map is used in this study beside other geological and geophysical information data from deep drilled wells. The Bouguer gravity anomaly map of the studied area was interpreted to delineate the subsurface structural pattern. The gravity data are intensively analyzed to extract information about the shallow and deep seated subsurface structures, to identify the main prevailing tectonic trends, to determine the depth to the near surface and basement rocks of this area and to reveal the most prospective areas for hydrocarbon accumulations. The depth determination to the high density, shallower and deeper subsurface sources is carried out by applying transformation techniques which include Euler deconvolution technique and power spectrum analysis method. The Bouguer gravity anomalies are separated into their regional and residual components by filtration method, first derivative method, horizontal and tilt derivative methods and continuation method. For quantitative interpretation of these subsurface structures, the 2.5-D gravity modeling technique was also applied for determination the depth to the basement rocks and determination of average densities of sedimentary cover. The calculated depth ranges between 2000 and 4000 meters. The residual gravity map shows that the area consists of successive positive and negative anomaly groups of horsts (anticlines) and grabens (synclines) tectonic features. The prevailing fault patterns present are as follows from strong to weak; E-W, N-S, NW-SE, ENE-WSW and NNE-SSW. The interpreted subsurface shallow structural map shows that the area had suffered from faulting cutting the subsurface rocks in E-W and NW-SE trends constituting a great number of horsts and grabens while the deep faults cuts the area in ENE-WSW and N-S deep faults forming great E-W sedimentary basins with great thickness which encouraged many oil companies to carry out extensively deep drilling for seven wells for oil and gas exploration inside the studied area. The study revealed the presence of a very large sedimentary basin at the west of this area and also revealed a huge straight anticline or igneous intrusion cutting the northern part in E-W trend with 90 km length and 5 km width.