Oil & Gas Well Log Interpreter

Processing Workflow

Step 1: Parse Input

Identify the input format and extract:

• Depth range and depth reference (MD or TVD; KB, DF, or GL)
• Available curves and their units
• Sampling interval (e.g., 0.125 m, 0.5 ft)
• Missing curve sections

If the input is a LAS file, extract the ~Well, ~Curve, and ~Parameter sections before reading data.

Step 2: Quality Check

Before interpreting:

• Flag bad-hole intervals: Caliper > [bit size + 2 inches or bit size + 5 cm]
• Flag washout-affected curves (RHOB, NPHI, SP unreliable in washed-out sections)
• Note any curve splices or tool changes
• Identify depth shifts or obvious spikes

Step 3: Shale Volume Calculation (Vsh)

Default method: linear GR index

IGR = (GR - GR_min) / (GR_max - GR_min)
Vsh = IGR  (linear)

If user provides a Larionov correction preference, apply it (Larionov, 1969):

• Tertiary/unconsolidated: Vsh = 0.083 × (2^(3.7 × IGR) - 1)
• Older consolidated rocks: Vsh = 0.33 × (2^(2 × IGR) - 1)

Note: Larionov corrections reduce Vsh relative to the linear GR index; choose the formula appropriate for the geological age of the formation.

State GR baseline values used:

• GR_min: clean sand baseline (typical: 15–30 API)
• GR_max: shale baseline (typical: 80–120 API in clastic systems)

Adjust baselines for carbonates: GR_min may be 5–10 API; shale GR higher.

Step 4: Porosity Calculation

Density porosity:

PHID = (RHOB_matrix - RHOB) / (RHOB_matrix - RHOB_fluid)

Default matrix densities:

Default fluid density: 1.0 g/cm³ (water), 1.1 g/cm³ (oil-based mud)

Neutron-density crossplot porosity:

PHIND = (NPHI + PHID) / 2   [for liquid-filled pores, average method]

Gas-corrected crossplot porosity (Bateman and Konen approximation):

PHIG = sqrt((NPHI² + PHID²) / 2)

Note: This is a simplified empirical correction suitable for moderate gas saturations. It tends to underestimate porosity in high gas saturation zones. For high-saturation gas sands, a more rigorous correction using gas PVT data is preferable.

Apply shale correction when Vsh > 0.1:

PHI_corrected = PHIND - (Vsh × PHI_shale)

Step 5: Water Saturation Calculation

Default: Archie's equation

Sw = ((a × Rw) / (PHI^m × RT))^(1/n)

Default Archie parameters for clastics:

For carbonates, use m = 2.0–2.5 (vuggy) or specify if dual porosity.

Rw estimation (if not provided): Use SP curve: Rw = Rmf × 10^(-(SSP / (60 + 0.133 × T))) Or use the water zone: Rw = RT_water × PHI^m × (1/a)

If Rw is unavailable, state it clearly and request the value from the user. Otherwise estimate from a water zone using Archie (Sw = 1 in clean water zone):

Rw = (RT_water × PHI^m) / a

Step 6: Pay Identification

Apply cutoffs to identify net pay intervals:

Flag if the user has specified different cutoffs.

Net pay = sum of intervals meeting all three cutoffs.

Step 7: Fluid Contact Identification

Look for:

• Sharp RT increase with constant or decreasing porosity → potential OWC or GOC
• NPHI/RHOB crossover (neutron less than density in pu units) → gas signature
• SP deflection changes
• Consistent Sw transitions

State contacts as:

• Definite: clear log response + pressure confirmation or test data
• Probable: log response alone, no independent confirmation
• Possible: ambiguous response, requires additional data

Output Format

Formation Evaluation Summary Report

# [Well Name] — 测井解释成果报告

## 1. 基本信息
| 项目 | 内容 |
|------|------|
| 井号 | |
| 解释井段 | m - m |
| 测井日期 | |
| 井眼尺寸 | mm |
| 钻井液类型 | |
| 钻井液密度 | g/cm³ |
| KB/DF 高程 | m |
| 数据来源 | |

## 2. 可用测井曲线
| 曲线 | 单位 | 井段覆盖 | 质量 |
|------|------|---------|------|
| GR   | API  |         |      |
| RT   | Ω·m  |         |      |
| RHOB | g/cm³|         |      |
| NPHI | v/v  |         |      |
| DT   | μs/ft|         |      |

## 3. 解释参数
| 参数 | 取值 | 来源 |
|------|------|------|
| GR_min (纯砂) | API | |
| GR_max (泥岩) | API | |
| 基质密度 | g/cm³ | |
| 地层水电阻率 Rw | Ω·m | |
| Archie a | | |
| Archie m | | |
| Archie n | | |

## 4. 分层解释成果表
| 层号 | 顶深(m) | 底深(m) | 厚度(m) | 岩性 | Vsh(%) | PHI(%) | Sw(%) | 解释 |
|------|---------|---------|---------|------|-------|-------|------|------|
|      |         |         |         |      |       |       |      |      |

解释符号说明：
- 油层：O
- 气层：G
- 油水同层：OW
- 气水同层：GW
- 水层：W
- 致密层/干层：D
- 泥岩：S

## 5. 储层物性汇总
| 储层 | 顶深(m) | 底深(m) | 有效厚度(m) | 平均孔隙度(%) | 平均Sw(%) | 流体类型 |
|------|---------|---------|-----------|------------|---------|---------|
|      |         |         |           |            |         |         |

## 6. 流体接触关系
| 接触面 | 推测深度(m) | 置信度 | 依据 |
|--------|-----------|--------|------|
| OWC (油水界面) | | | |
| GOC (气油界面) | | | |

## 7. 解释评价与建议
[综合评价储层质量，推荐射孔井段，提示需进一步确认的不确定点]

## 8. 数据缺口与补充建议
[列出缺少的参数、需要的化验分析、或建议加密的测井项目]

Lithology Identification Rules

Use the following log response patterns to classify lithology:

Special Interpretation Notes

Gas Effect

When NPHI < PHID (density porosity > neutron porosity — a "crossover" in standard overlay):

• Indicates gas in pores
• Apply gas correction: PHIG = sqrt((NPHI² + PHID²) / 2)
• RT should be high; GR should be low (clean gas sand)

Carbonate Reservoirs

• Secondary porosity (fractures, vugs) not captured by matrix logs alone
• FMI / image logs needed to characterize fracture porosity
• Archie cementation exponent m may vary 1.8–3.0 for vuggy carbonates
• Dual porosity systems: total porosity ≠ effective porosity

Tight / Low-Permeability Reservoirs

• Low porosity (< 8% for sandstone, < 3% for carbonate)
• RT may be elevated even in water-bearing zones due to low saturation
• NMR T2 cutoff critical for permeability estimation
• DT / sonic porosity helps cross-check

LAS File Parsing Instructions

LAS 2.0 format sections:

~Version      → LAS format version
~Well         → Well name, coordinates, dates
~Curve        → Column definitions (mnemonic, units, description)
~Parameter    → Formation parameters, mud properties, Rw
~A (or ~ASCII)→ Numerical data columns

When parsing:

1. Extract well metadata from ~Well
2. Map column names from ~Curve to their positions in ~A
3. Handle null values (default: -999.25 or -9999.0) — replace with NaN before calculations
4. Note depth reference: STRT, STOP, STEP in ~Well section

Quality Flag Table

Include a quality flag column in the output table:

Calculation Transparency

Always show:

• The formula used for each parameter
• The input values substituted
• The result with units
• The source of constants or parameters

Example:

Sw at 2456.5 m:
  PHI = 0.18 (from RHOB = 2.44 g/cm³, matrix = 2.65)
  RT = 8.2 Ω·m, Rw = 0.05 Ω·m
  Sw = ((1.0 × 0.05) / (0.18² × 8.2))^(1/2) = 0.41 = 41%
  → Oil pay (Sw < 65% cutoff)

Output Checklist

Before delivering results, verify:

• All curve mnemonics identified and units stated
• Bad-hole intervals flagged
• Vsh method and baselines stated
• Porosity method stated, gas correction applied if crossover present
• Rw source stated
• Archie parameters stated
• Pay cutoffs stated
• Fluid contacts noted with confidence level
• Interpretation table populated with quality flags
• Missing data or parameter gaps listed

Example Prompt Triggers

This skill should activate for prompts such as:

• "帮我解释这段测井数据，判断是油层还是水层"
• "这是LAS文件内容，帮我算孔隙度和饱和度"
• "GR 85API，密度 2.3，中子 28%，电阻率 35 欧姆米，这是什么层？"
• "从这个测井表格中识别出所有储层并计算净油层厚度"
• "Identify pay zones from this log suite"
• "Calculate Sw using Archie's equation for these intervals"
• "帮我写这口井的测井解释报告"
• "Interpret this LAS file and give me a formation evaluation"