Surface Geology And Subsurface Geology

1. Mehran University Of Engineering
Technology (SZAB) Campus

2. CONTENTS
1. Surface Geology
2. Subsurface Geology
3. Well Cutting
4. Coring
5. Logging While Drilling
6. Formation Testing
7. Wireline Well-Logging Technique

3. Surface Geology
 There are several areas to look for oil. The first
is the obvious, on the surface of the ground.
Oil and gas seeps are where the petroleum
has migrated from its’ source through either
porous beds, faults or springs and appears at
the surface. Locating seeps at the surface
was the primary method of exploration in the
late 1800’s and before.

4. Subsurface Geology
 What is Subsurface Geology?
Subsurface geology is the study of the physical
properties and location of rock and soil found
below the ground surface.
• One of the most valuable reasons for learning
about the subsurface is understanding the
materials below man-made structures.
• These studies are commonly called geotechnical
reports and are critical for building structures
safely in areas with geologic hazards.

5. Well Cutting
 Well samples are produced from drilling operations,
by the drill bit penetrating the formation
encountered in the subsurface.
 Samples are taken at regular intervals. They are
used to establish a lithologic record of the well and
are plotted on a strip sample log.

6. Coring
 One way to get more detailed samples of
a formation is by coring, where
formation sample is drilled out by means
of special bit.
 This sample can provide:
 Detailed lithological description.
 Porosity, permeability, fluid
saturation and grain density.
 These parameters are measured in the
laboratory and serve as a basis for
calibrating the response of the porosity
logging tools and to establish a
porosity/permeability relationship.

7. Logging While Drilling
 One of the major drawbacks of wireline
information is that it is received several hours to
several weeks after the borehole is drilled.
 During this time period, the formation can undergo
significant alteration, especially in its fluid
saturation, effective porosity, and relative perm.
 LWD allow wire line-type information to be
available as near as real-time as possible.
 Logging While Drilling (LWD) is a technique of
conveying well logging tools into the well borehole
down hole as part of the bottom hole assembly
(BHA).

8. Logging While Drilling
 Some available measurement in LWD technology:
 Gamma Ray
 Resistivity
 Density
 Neutron
 Sonic (fairly recent)
 Formation pressure
 Formation fluid sampler
 Borehole caliper (Ultra sonic azimuthal caliper,
and density caliper).

9. Formation Testing
 Is a means of obtaining information concerning the
liquid and pressure in an open-hole formations.
 Three methods:
■ Wire line testing
■ Drill stem test (DST)
■ Well Test Analysis

10. Methods of Formation Testing
 Wire line testing:
 Provide reservoir fluid samples, reservoir pressure,
an indication of fluid mobility and information on
reservoir continuity.
 Drill stem Test:
 A drill stem test (DST) is a procedure for isolating
and testing the surrounding geological formation
through the drill stem.

11.  Well Test Analysis:
 Two types of testing: pressure build-up and draw
down test.
 The primary objectives of well testing are to
establish:
 Permeability thickness and permeability (K)
 Stratification (by sequential testing of layer).
 Well productivity.
 Investigate reservoir boundaries and size.

12. Wireline Well-Logging Techniques
 Wireline logging involves the measurement of various
properties of a formation including electrical resistivity,
bulk density, natural and induced radioactivity, hydrogen
content and elastic modulae.
 These measurements may then be used to evaluate not
only the physical and chemical properties of the
formation itself, but also the properties of the fluids
that the formation contains.

13.  There are open hole logs and cased hole logs.
 The open hole logs are recorded in the uncased portion of
the wellbore. Cased hole logs are recorded in the
completed or cased well.
 There are measurements that can be made in both the
open and cased holes and some that can only be made in
open holes. Resistivity and density porosity are two
examples of measurements that can be made in an open
hole but not in a cased hole.

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