CHAPTER 06
CHAPTER 06
THE SUBSURFACE
THE SUBSURFACE
ENVIRONMENT
ENVIRONMENT
1
1
-
-
GROUND WATER AND
GROUND WATER AND
TEMPERATURE
TEMPERATURE
1.1 – GROUND WATER
1.1.1 – Origin of ground water
1.1.2 – Chemistry of ground water
1.2 – TEMPERATURE
1.2.1 – Subsurface Temperature
1.2.2 – Regional thermal Variations
1.2.3 – Local thermal Variations
1.1
1.1
-
-
GROUND WATER
GROUND WATER
1.1 .1 – Origin of ground water (GW)
04 types of GW
1. Meteoric water.
2. Connate water.

3. Juvenile water.
4. Mixed water.
04 Types of GW
04 Types of GW
Meteoric water
Infiltration of rainwater.
Distribution @ shallow depth.
Total mineralization: Low
Tens to be Oxidizing
pH: Often acidic due to dissolved humic, carbonic
and nitrous acids.
Connate water
Ancient sea water which was trap in the sediment
during burial.
Differs from seawater both in concentration of
dissolved salt and pH, and Eh .
04 Types of GW (cont.)
04 Types of GW (cont.)
Juvenile water
Primary of magmatic origin.
Brought to near –
surface environment dissolved in
magma.
Usually mixed with either connate or meteoric
water.
Mixed water
Results from the commingling of meteoric, juvenile
and connate waters.
Usually between the near –
surface meteoric water,

juvenile and the deeper, more saline connate water.
1.1 .2
1.1 .2
–
–
Chemistry of ground water
Chemistry of ground water
Connate water, meteoric water and mixed water can
be differentiated on the basics of their chemistry.
Way can be done:
First:
Eh: Oxidation/reduction potential and
pH: Measure of acidity or alkalinity of the water
Eh >0: Considered to be oxidizing
<0: Considered to reducing
pH = 7: [H
+
]= [OH
-
] Considered to be neutral
<7: Acidic
> 7: Alkaline
Fig 01
Deep connate water show a wide range of Eh and pH
depending on their history and how much they’
ve
mixed with meteoric water.
Oilfield brines
tends to be more alkaline and more
strongly reducing than seawater.

The Eh and pH of pore fluids control the precipitation
and dissolution of cements such as the carbonates and
ion oxides, as well as the alterations of clays minerals
in subsurface rocks 
Extremely important to
understand the relationships of Eh and pH to
diagenesis and the evolution of porosity.
Chemistry of ground water (cont.)
Chemistry of ground water (cont.)
Second: Salinity
In general salinity of GW increases with depth
(normal hydrochemical profile)-
Fig.02. The rate of
increases varies from basin to basin, even from place
to place within a particular basin.
Typical seawater has a salinity of about 35ppthousand
(3.5%).
The salinity of GW range from near zero (in newly
introduced meteoric to > 600ppthousand (60%) in
connate water within evaporate formation.
Fig 02
Reversal hydrochemical profile
have been observed
due to two possible causes:
1. Meteoric can be trapped beneath an
unconformity and preserved as “Paleoaquifer”
with
relative low salinity as compared connate water above
the unconformity.
2. Overpressure: In shale sequences, formation

water is trapped.
In shale, the increases in salinity with depth is less
noticeable than in sandstones: Water moves upwards
in compacting sediments, shale acts at semipermeable
membranes preventing salt escaping from the sands.
Four major sub. environment:
1. Zone 1 (surface → 1km) uniform Zone of
circulating meteoric water. Salinity fairly uniform;
2. Zone 2 (1 → 3km) gradually increases with depth
Saline formation water is ionized;
3. Zone 3 (3km) Chemically reducing
environment, in which hydrocarbons form. Salinity
uniform with increasing depth; may even decline
if overpressured;
4. Zone 4 incipient metamorphism with
recrystallization of clays to micas.
Oder Catelogy Total dissolved Solids
g/l %
1 Super fresh < 0.2 0.02
2 Fresh 0.2 – 1.0 0.02 – 0.1
3 Brakish water 1.0 – 3.0 0.1 – 0.3
4 Light saline 3.0 – 10 0.3 – 1.0
5 Saline 10 – 35 1.0 – 3.5
6 Brine > 35 > 3.5
Klimentov, 1977
Table 01
Oder Catelogy Total dissolved Solids
(mg/l)
1 Fresh < 1,000
2 Brakish water 1,000 – 10,000

3 Saline 10,000 – 100,000
4 Brine > 100,000
Freeze and Cherry, 1979
Table 02
°
Regional isosalinity maps are very useful
exploration tools.
Area of high salinity possible indicate stagnant
regional are uneffected by meteoric flushing
(where oil/gas accumulation may be preserved)
Composition
Composition
° Meteoric water differs from connate water both
salinity and proportions of dissolved irons.
° Meteoric water divided to:
+ High proportions of SO
2-
4
and Na
+
.
+ High CO
2-
3
and Na
+
.
Connate water divided to:
Connate water divided to:
+ High proportions of CL

-
and Mg
2+
.
+ High CL
-
and Ca
2+
.
° In comparison to seawater, connate water high
concentration of soluble chlorine and sodium.
(Tab 03)
(Br more abundant than is seawater)
° SO
2-
4
in connate water << seawater:
+ Precipitation of CaSO
4
+ SO
2-
4
reduction by bacterial action, producing
H
2
S gas.
Table 03
° Depletion of Mg
2+
in connate water due to

dolomitize .
° Ca
2+
in connate water > in seawater:
+ Release of calcium from dolomitize.
+ Dissolution of SO
2-
4
.
+ Release of calcium during clay diagenesis
(montmorillonite + K
+
→ illite + Ca
2+
+ Na
2+
+
H
2
O).
+ Low rate of calcium precipitation.
• Depletion of potassium probably results from the
uptake of that element by clay minerals.
• Connate waters also contain traces of dissolved
hydrocarbons which are not common in normal
sea water (Buckley et al., 1958).
– This is significant for two reasons. First, it raises the
possibility of regionally mapping dissolved
hydrocarbons as a key to locating new oil and gas
fields. Second, it has some bearing on the migration of

both oil and gas.
GW research application in O&G Exploitation &
Exploration
(By Tran van Xuan)
• Các mẫu nước mỏ dầu đạt yêu cầu được phân tích để xác
định:
– Tổng độ khoáng hóa, một số nguyên tố, ion (Cl
-
, SO
4
2-
, HCO
3
-
,
Na
+
& K
+
, Mg
2+
, Ca
2+
…).
– Quan hệ giữa các ion.
– Xác lập một số quan hệ tỷ lệ, phân loại theo Sulin.
– Đánh giá sự thay đổi của độ tổng khoáng hoá theo chiều sâu.
• Ngoài ra một số mẫu nước còn được tiến hành phân tích
hàm lượng vi nguyên tố như I, Br, Sr,….
• Tính toán khả năng sa lắng của canxit, thạch cao và sinh

khí CO
2
tự do .
• Đánh giá nguồn gốc, quá trình biến đổi của nước các mỏ.
GW research application in O&G Exploitation &
Exploration (Cont.)
• + Phân loại Xulin: Phân loại của Xulin dựa trên cơ sở
phân chia các loại nước theo các tỉ số nhất định của các
ion, đặc trưng cho các điều kiện thành tạo khác nhau
của nước dưới đất nói chung và đặc biệt với nước dưới
đất trong các vùng mỏ dầu khí; vì vậy phân loại này
được sử dụng rộng rãi trong địa chất thuỷ văn các mỏ
dầu khí.
• Trên cơ sở xem xét các mối quan hệ (Trong đó rNa
+
,
rCl
-
… được tính bằng %đl/l):
•


rCl
rNa
2
4




rSO
rClrNa
2


rMg
rNarCl
• Xulin chia nước DĐ thành 4 loại:
• 1. Loại nước sunphat natri có nguồn gốc rửa lũa đại lục, được
đặc trưng bằng:
• 2. Loại nước bicabonat natri (nước kiềm) có nguồn gốc đại
lục, khí quyển được đặc trưng bằng:


rCl
rNa
>1;
2
4



rSO
rClrNa
> 1 và
2


rMg
rNarCl

<0


rCl
rNa
>1;
2
4



rSO
rClrNa
< 1 và
2


rMg
rNarCl
<0
• 3. Loại nước clorua magie liên quan với nguồn gốc biển và
được đặc trưng bằng:
• 4. Nước clorua canxi (nước cứng) có nguồn gốc biến chất sâu
(liên quan với các mỏ dầu khí) được đặc trưng bằng:
•
> 1 ;
> 1 và <0
• Phân loại Xulin được biểu diễn bằng hai hình vuông cùng đặt
theo một đường chéo. (Fig.03)



rNa
rCl
> 1 ;
2


rMg
rNarCl
<1 và
2
4



rSO
rClrNa
<0


rNa
rCl
2


rMg
rNarCl
2
4




rSO
rClrNa