Perhaps look at surface outcrops of the same zone elsewhere along trend?

A: Thickness constraints would be a start.

B: Area might have clues from surface features...such as River drainage patterns

From A & B above perhaps estimate from that an absolute minimum Volume that it could be, then an maximum ...and average the two? 

Just free thinking

What you can do is to calculate the boundary of a basin using gravity data and definitely not the source rocks which are sedimentary rocks. So you should use appropriate models for your estimations and this would give you the areal extent and volume.

Dear Hasan!

To determine the prospects of the region with the gravimetric data necessary to know the density of the horizons of both vertically and horizontally. Knowing the distribution of the density using the formulas and paletok can calculate the gravitational effects which can judge about the prospects.

Steps

(1)Calculate corrected  Bouguer Gravity data.

(2) Separate Regional and Residual gravity data.

(3) Take second derivative of Residual data.

(4) The zero contour of the second derivative data will give outer boundary of the causative body. The hydrocarbon productivity of the causative body need to be matched with anomaly obtained from seismic and source rock study.