Original Oil in Place
Volume of oil originally in a reservoir (in stock-tank barrels).
Formula
Variables
Example
A=640, h=50, φ=0.2, S_w=0.3, B_o=1.2: OOIP ≈ 28.9 MMSTB
How it works
Original oil in place (OOIP) estimates how much oil was initially contained in the reservoir rock before production began. The term 7758 converts acre-feet of reservoir volume into stock-tank barrels, while A h gives the gross bulk rock volume, φ gives the fraction that is pore space, (1 − Sw) removes the water-filled part of that pore space, and Bo converts reservoir-condition oil volume into surface-condition barrels. In other words, it takes the rock volume, extracts only the oil-bearing pores, then adjusts for oil shrinkage as pressure and temperature change from reservoir to stock-tank conditions.
When to use it
Use this formula in reservoir engineering when estimating the original oil volume for volumetric reserves calculations, field screening, or sanity-checking a reservoir model. It is especially useful early in field life, before production data are sufficient for strong material-balance estimates.
Worked example, step by step
Given: A = 1,200 acres, h = 35 ft, φ = 0.18, Sw = 0.25, Bo = 1.30 Step 1: Compute gross reservoir rock volume A h = 1,200 × 35 = 42,000 acre-ft Step 2: Compute pore volume A h φ = 42,000 × 0.18 = 7,560 acre-ft of pore space Step 3: Remove water-filled pore space A h φ (1 − Sw) = 7,560 × (1 − 0.25) = 7,560 × 0.75 = 5,670 acre-ft of oil-filled pore volume Step 4: Convert to stock-tank barrels OOIP = 7758 × 5,670 / 1.30 OOIP = 43,994,760 / 1.30 = 33,842,123 STB Final answer: OOIP ≈ 33.8 MMSTB
Common mistakes to avoid
- Using total porosity instead of effective porosity when the reservoir has significant non-contributing pore space
- Forgetting that Sw must be the water saturation in the oil zone, not the average saturation across the whole reservoir
- Mixing units by using square feet or meters for area without adjusting the 7758 conversion factor
Did You Know?
The constant 7758 is the number of barrels in one acre-foot of rock.
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