Quantitative evaluation

1. Quantitative evaluation Examine procedures in which data from 2 or more logs will be used to evaluate for the amount of hydrocarbon compared to pore water within a formation The proportion of pore space occupied by water is known as water saturation (Sw) e.g. if 65% of the formation is occupied by water and 35% by hydrocarbons the water saturation is 0.65 (percent water saturation is 65%) Definition of the formation factor (F) F = Ro/Rw Where Ro is the resistivity of the total formation (rock matrix + fluids) saturated with water Rw is the resistivity of the water itself

2. F= Formation factor is used to help calculate water saturation – function of porosity • Definition F = Ro/Rw • Definition for water saturation Sw = √RO/RT • RO = RT (for a 100% saturated sand) • Substituting in the equations and rewriting for water saturation we have: • RO = F. RW • SW = F. RW/RT

3. QUANTITATIVE EVALUATION FOR HYDROCARBON POTENTIAL Archie equation (1942) … F = a/φm a = constant which is lithology dependent m = empirically derived constant φ = porosity (e.g. 30% enter as 0.30) Actual values for a and m are lithology and region dependent Some examples F = 0.82/φ uncompacted sands F = 1/ φ2 compacted formations, chalk F = 1/ φ2.2 compacted clastic and oolitic rocks F = 0.62/ φ2.15 recrystallized carbonates (sucrosic texture) F = 1.13/φ1.73 generic value for large number of samples

4. Analysis for Hydrocarbon Potential • Definition: Rwa = Rt / F • Rwa = apparent water resistivity • Rt is measured from a deeply focused induction log • F is calculated from the formula using log measured porosity value • est Sw (%) = √Rw/ Rwax 100 • Rw = Rwa of a 100% water saturated formation. Measure from an obvious water-bearing formation • Rwa Analysis Rwa/ Rw • Ratio Rwa < 2 No oil production • Ratio Rwa > 4 Oil production expected

5. Hydrocarbon potential Use F = 0.83 / φ2

6. READINGS AND CALCULATIONS