A study of the capability of the finite element method in gravity anomalies separation of oil traps



This paper attempts to investigate the capability of the numerical finite element method for the removal of the regional effects from the observed gravity data from the Dehnow anticline.
From a geological point of view, the Dehnow area is a part of the Fars sedimentary basin south-east Iran. The Khamy formation and Bangestan group are the oldest geological structures in the area that have outcrops. Younger structures consist of Aghajary, Mokhtari, Mishan, Gachsaran and Asmary. The dominant structural trend in the area is northwest-southeast. The Dehnow anticline is located between the Hendurabi and Razak faults. These faults are almost perpendicular to the Dehnow anticline. The presence of these two series of perpendicular faults has been well proved by the detailed geological investigations carried out in the area. The first type of the faults with a northwest-southeast direction has formed oil seeps due to the extension of these types of faults to the dipper layers. The second types of faults in the salt layers of the Hormoz formation at the base of the Dehnow anticline may be related to the salt intrusion. The evidence of the salt outcrops can be recognized at two points from the Dehnow anticline.
The Dehnow anticline is surrounded by Ashkenan, Ahal, Boochir, Hamiran, Hashniz and Kemeshck towns. The Tabnack gas structure is located in the west of this district. The anticline can be accessed through Asalouie-Bandarlengeh and Lamard-Ashkenan- Gavbandy roads. The area has a very harsh topography with numerous mountains and valleys. The region is very warm and wet in the summer. It has a mild winter.
A careful geological study of the area, a detail investigation of structural features such as faults associated with the Dehnow anticline and application of the proper geophysical techniques and other exploration methods is necessary to investigate its subsurface extension and to further identify salt plug intrusion into this anticline. Furthermore, the information obtained by the various exploration techniques can help to design a cost management program related to any further investigation in the area.
The gravity method has been used in the past in many applications related to petroleum exploration studies as a way of investigating oil traps and exploring the impact of faults and intrusions in particular salt into such geological structures. This method together with other geophysical methods including magnetic and Magnetotelluric (MT) have been used in the study area to achieve the above objectives.
Anomaly separation using appropriate techniques is noted to be an important stage in the interpretation of gravity data in order to accurately detect the subsurface oil traps. The literature review has shown that despite the conventional separation methods such as first and second vertical derivatives, upward and downward continuations, weighting average and trend surface analysis that have been used during the past few decades in the separation of gravity anomalies in the exploration of oils and ore deposits, the application of the finite element method related to anomaly separation has not been widely reported so far. Using new techniques in the regional-residual anomalies separation of the observed gravity data is a crucial demand in gravity interpretation.
In this paper, the finite element approximation method was used to remove regional effects from the measured gravity data of the Dehnow anticline. For regional gravity approximation, four nodes element, eight nodes isoparametric quadratic element and twelve nodes cubic element were used. The necessary computations were performed on a non-dimensional space, ranging between -1 and 1 and subsequently translated to the real x-y space.
The capability of the finite element method in regional-residual separation of the gravity data was evaluated using the conventional separation techniques including the trend surface method. The residual map obtained using the finite element method indicates the presence of two anticlines and three syncline structures with a northwest – southeast strike. The anticlines identify the Dehnow structure. The results of the finite element method are in close agreement with those results obtained from the conventional methods; confirming that the finite element approximation technique is a capable method for computation of the regional gravity anomaly free from residual effects.