Introduction to Geology

Geology is a core science that underpins much of what we do in the Oil & Gas business. A good understanding of the principles of Geology as they apply to Petroleum is essential for any subsurface discipline.

The primary objective of this course is to introduce E&P professionals to the key concepts and principles of Geology as applied to the Oil & Gas industry. The course will provide a summary of the fundamentals of Geology that need to be understood in order to integrate such information in the processes of petroleum exploration, development, and production.

The course is classroom-based, containing a mix of theory, application, exercises and videos.

Introduction

Course Objectives - History and Economics of Petroleum - Prudhoe Bay Example

Geology in General

Geological Principles - Earth Structure and Plate Tectonics - Geological Time and Age Dating - Major Rock Types

Structural Features

Structural Style and Stress Fields - Folds, Faults, Unconformities, and Fractures - Structural and Hydrocarbon Traps

Clastic Depositional Systems

Continental Systems – Fluvial Systems - Deltaic Systems and Types - Marine Systems – barriers and deepwater fans

Carbonate Depositional Systems

Carbonate and Clastic systems contrasts - Carbonate Facies Models - Carbonate Platforms and Ramp Systems - Carbonate Reservoir Properties - Classification of Carbonate Rocks

Geologic Mapping and Cross-Sections

Mapping and Contouring Concepts - Contouring Types and Guidelines - Isopach / Net Pay Mapping Exercises - Structural and Stratigraphic Cross-Sections - Constructing a Cross-Section - Property Mapping Exercises - North Frisco City Field Example

The Petroleum System

What is a Petroleum System? - Processes of Hydrocarbon Generation - Source Rocks – Organic Matter Types - Traps and Seals – definition and types - The Play Concept - Petroleum System Processes and Events

Correlation and Stratigraphy

Principles of correlation - Use of Wireline Logs for Correlation - Correlation Approaches – Lithostratigraphy and Chronostratigraphy - Sequence Stratigraphy, Seismic Stratigraphy & Biostratigraphy

Pore Systems and Diagenesis

Characterizing the Pore System - Porosity and Permeability - Classification of Clastic Rocks - Creation, destruction and preservation of porosity - Diagenesis - Clay Types and Distributions - Porosity types in clastic and carbonate rocks

Geological Modeling

Building a Static Reservoir Model - Input Datatypes - Reservoir Heterogeneity and Scales of Investigation - Deterministic and Stochastic Modeling - Using Seismic Data in Modeling - Derivation of Geological Models using Flow Units

Unconventional Resources

Shale as a hydrocarbon source and reservoir - Unconventional Gas and Oil - History, definitions, economics.

Seismic Methods and Petroleum Geology

Use of Seismic Tools in Petroleum Geology - Basic Principles of the Reflection Seismic Method - Data Acquisition – Land, Marine, and Borehole - Data Processing - Seismic Interpretation – 2D, 3D, and 4D

Wellbore Data - Wireline and Core

What Logging Means - Different Measurements We Make - Basic Wireline Tools, and what they measure - Processing and Interpretation - Analysis of Porosity, Saturation, and Lithology - Coring a well, Conventional and Sidewall - Core Analysis - Log-Core Correlation and Calibration.

Operations and Wellsite Geology

This course provides a complete overview of wellsite operations from the perspective of the Operations Geologist and the Wellsite Geologist. The focus is on being able to understand the job functions that are typically performed at the wellsite, and what use is made of the large amounts of data collected. The course will also provide an overview of essential drilling operations that have a direct bearing on these discliplines.

Wellsite geologists study rock cuttings from oil and gas wells to determine what rock formations are being drilled into and how drilling should proceed. They identify critical strata from core samples and rock-cutting data and build up knowledge of the structure being drilled. They are experienced geologists, deciding when specialized tests should be carried out and, ultimately, when to stop drilling. They send reports and logs of completed drilling to the operations geologist and offer geological advice to oil company representatives. They also incorporate health and safety requirements in daily geological operations. Wellsite geologists also liaise with drilling engineers, petroleum engineers and mudloggers during the course of projects.

Participants will learn the techniques used by wellsite geologists in formation evaluation through a combination of lectures and exercises that can typically be done at the wellsite. At the end of the course, the participants should have a good understanding and knowledge of the requirements of both Operations and Wellsite Geology.

The course will blend classroom instruction with several practical exercise sessions. If location permits, a visit to a wellsite can be arranged.

Introduction

What is an Operations and Wellsite Geologist?

Exploration and Drilling Programs – Risk assessment, regional analysis, pre-drill data acquisition.

Components of a prospect

Overview of Petroleum Geology

The Petroleum system – elements & processes

The Reservoir – Sedimentary Environments

The Trap – Structure & Stratigraphy

Mapping and Cross-Sections

Pore Systems and Flow Units

Reserves and Resources – classification and categories

Data Types and Management

Wireline Data – open and cased hole, testing, LWD and MWD

Mud Logging Data – geological, drilling, pressure

Core Data – whole core and sidewall core

Drilling Operations

The drilling team – who does what?

Types of Drilling rigs

Rig Sub-systems – power, hoisting, rotary, circulating, well control

Drilling tools and components – including drilling fluid

Well control – kicks causes – basic calculations – safety equipment – kill methods

Well costs

Planning a Well

Well Design

Directional Drilling – methods and calculations

Geosteering

Mud Logging

The Logging unit – components and functions

Services – monitoring, sampling, analysis

Cuttings analysis and description

The mud log

Safety considerations – monitoring, overpressure, downtime

Gas detection and analysis – types of gas – gas shows – equipment and methods

Coring and Core Analysis

Coring methods and equipment

Whole Core and Sidewall Core

Core handling and preservation

Basic calculations – core-log integration

Pore Pressure and Wellbore Stability

Overburden and compaction

Pore pressure generation – estimation – normal and abnormal pressure

Detection from Seismic – pre-drill prediction

Stress and Strain – wellbore failure – lost circulation

Sampling – types and preservation

Quality control of acquired wellsite data

Wellsite Geologist Responsibilities

Wireline Logging Tools and Measurements

Review of basic logging tools for lithology, porosity, saturation

Resistivity and Invasion

Wireline Log Interpretation

Basic concepts – quicklook workflow

Determination of lithology

Shale – calculation of shale volume - effects and corrections

Determination of porosity

Determination of water saturation – resistivity effects – formation water

Analysis techniques - crossplots

Cased Hole Logging

Production Monitoring and Reservoir Performance

Cement Bold Log Evaluation

Borehole Imaging

Dipmeter Principles

Dipmeter Interpretation

Structural Geology for Petroleum Applications

This course is delivered in 3 hour segments over 3 days. The course is delivered by virtual means using Skype for Business.

This Virtual Classroom has been designed to familiarize class participants with the fundamentals of Structural Geology, as used and required in and for the Oil & Gas Industry.
Structural Geology is defined as “The discipline within the science of Geology that relates to rock unit geometry and the deformational histories that produced those geometries.”

Session One (3 hours) - Structural Concepts and Fundamentals

Forces - Stress and strain

Tectonics - Extensional, Compressional, Strike-Slip

Scale - Regional to Microscopic

Students will gain an understanding of the forces involved in the creation of structure, and of the relationship between those forces and the subsequent structure type. This will be discussed at all scales, from scall-scale to regional. The tectonic origin of the forces will be discussed in relation to the large-scale structures that result. At the end of the session, students will have a clear understanding of how and why structures happen.

Session Two (3 hours) - Expression of Structure

Ductile Deformation - Folding

Brittle Deformation - Faulting

Brittle Deformation - Fracturing

This session is devoted to understanding the types of structures that can be formed, how they are formed, and how to recognise and describe the structure. The three basic categories - folding, faulting, and fracturing - will be covered, with respect to their relevance to the petroleum industry and their ability to create petroleum traps. At the end of the session, students will understand how to recognise and classify structure.

Session 3 (3 hours) - Interpretation of Structure

Seismic Interpretation of Structure

Structural Interpretation of Borehole Images

Surface Representation of Structure

This session is devoted to the interpretation of structure from the data meansurements and types currently employed in the petroleum industry. This is divided into Geophysical, Borehole (Petrophysical) and Mapping sections. At the end of this session, students should be able to perform functional interpretations from these types of data.

Integrated Reservoir Modeling: Interpretation, Evaluation, and Optimization with Petrel

This five day interactive and practical course will help participants to use geological modeling to produce realistic volumetric estimations for hydrocarbon reservoirs. It will show how new operational data and revised interpretations, such as a new well or a recently discovered fault, can be identified and incorporated into models at any point in the workflow.

This course will introduce the procedures and workflow for building a 3D model, regardless of the software available to the modeling team. In other words, this will be an attempt at a best practice approach to a complex and varied workflow. There will be a particular emphasis on integrating static and dynamic reservoir properties with the geological facies model. Exercises will be done using industry standard modelling software (Petrel), although it should be emphasized that the purpose of the course is methodology and not software operation.

Relevant industry case studies and practical applications will be reviewed throughout the course. Participants will gain an understanding of the key challenges associated with building effective 3D reservoir models from interpretation and design to quality assurance and optimization of results.

Participants will understand the science and workflows behind building consistent 3D reservoir models including fluid distribution, permeability, compartments and volumetric estimation. They will learn how integrate data from cores and logs and how to upscale this data into geological and flow simulation models that will have a high impact on field development and production scenarios.

Conceptual Design and Workflow

Reservoir envelope: top and base structure
Internal framework: correlation scheme
Reservoir compartments: fault geometry
Reservoir architecture: facies model
Petrophysical property distribution
Volumetric Assessment

When planning a reservoir modeling project, the ultimate purpose of the model must be defined. To this end the geological model should address all of the above points. On the first day, participants will learn about the retention of relevant fine-scale detail through upscaling. The process of building a 3D reservoir model will always follow the same general workflow, regardless of the tools available to the modelling team. Each of the steps will be outlined in the course with an appreciation of the required input data, associated uncertainties, and likely deliverable and its use. The database used for the exercises will be from a real field that includes two different clastic reservoir types with different modeling challenges.

Reservoir Framework

Depth conversion uncertainty
Model surface selection and quality control
Fault modelling and compartments
Stratigraphy and correlation
Grid construction

In volumetric models the greatest uncertainty is usually the gross rock volume at the top structure map and the hydrocarbon contact. Depth conversion, where well data can be sparse, leads to much of this uncertainty. Structural models that result in over complexity, much of which cannot be modelled, may drive seismic interpretation. Participants will learn how to decide what structural elements to include, which can be a source of much debate. Likewise too much well-to-well correlation can over science a model especially when still in development and drilling surprises are common, so it will be covered on this day as well.

Reservoir Architecture

Depositional models and facies analysis
Core-log integration
Basic statistics
Objects and indicators
Seismic conditioning
Facies modelling

Participants will learn about the different clastic and carbonate depositional environments and how to best characterize them for subsequent modelling. The dependence on statistics will be introduced as a way to demonstrate the different methods of facies modelling available.

Property Modeling in 3D

Basic petrophysics
Rock typing
More basic statistics
Porosity models
Saturation models
Permeability models

Running through the course will be the theme of petrophysics, always calibrated to the geology, as a way to distribute reservoir properties. Participants will be shown simple rock typing methods that are readily applicable to 3D models. Particular attention will be paid to saturation height methods that accurately distribute fluids through the model.

Uncertainty and Upscaling

Geological model analysis
Hydrocarbon volumes initially in place
Drainable volumes
Simulation grid construction
Property upscaling
Multiple scenarios, realizations, and ranking

What makes a good static model will be discussed. The methods of interrogating and analyzing the results described before addressing the thorny question of upscaling for dynamic simulation will also be covered.

Basin Analysis and Petroleum Systems Modeling

The quality of a numeric computer model is highly dependent on the quality of the input data. This 5-day course covers the key aspects of basin analysis and subsequent basin and petroleum systems modeling from input to output. We will discuss basin evolution beginning with plate tectonics, all the way to petroleum generation and migration. We will look at how risks and uncertainties influence our understanding of the petroleum systems within a basin and how we can quantify those uncertainties.

The aim of this course is to provide the basic geoscience background needed by anyone engaged in Petroleum Systems Modeling. Attendees will learn what kinds of questions to ask, what kind of data is needed to build models and solve particular problems and to apply geological reasoning to quantifying uncertainties.

Introduction to Sedimentary Basins

Definitions
Plate Tectonics

Basin Forming Mechanisms

How are basins formed?
Where are basins formed?

Basin Classification and Structural Analysis

Nomenclatures
Structural styles within sedimentary basins

Basin Fill

Sequence Stratigraphy
Depositional Environments

Geochemical Analysis

Organic Matter in Sediments
Source Rocks
Geochemical Analysis for Petroleum Exploration

Temperatures in Sedimentary Basins

Pressure and Compaction
Heat Flow
Temperatures

Petroleum Systems Modeling

Hydrocarbon Generation
Hydrocarbon Migration
The Petroleum System Approach
Uncertainty Management and Quantification

Petroleum Systems Modeling

Hydrocarbon Generation
Hydrocarbon Migration
The Petroleum System Approach
Uncertainty Management and Quantification

Carbonate Sequence Stratigraphy and Application to Petroleum Reservoirs

This course examines the processes leading to the formation and accumulation of carbonate rocks and how sequence stratigraphy provides a predictive method to understand the formation of carbonate reservoirs.

Facies and facies models are described, including their variation in different tectonic settings. The principles and the practical application of sequence stratigraphy are studied, together with discussion on how this improves our understanding of carbonate reservoirs.

Diagenetic changes that carbonate rocks undergo are also analysed, and, where possible, set within a sequence stratigraphic framework.

Processes and environments of carbonate sediment formation. Carbonate factories. Depositional environments, facies for attached carbonate platform; Florida Shelf.
Exercises: Modern carbonate grains and facies; the building blocks for limestones
Depositional environments and facies of unattached carbonate platforms and Bahamas carbonate slopes. Facies models for rimmed carbonate platforms.
Exercises: Caicos platform; environments and facies, Sedimentary (graphic) log construction

Depositional environments, facies and facies models for carbonate ramps. Arabian Gulf Case Study
Exercise on graphic log and cross section construction and interpretation
Principles of carbonate sequence stratigraphy.
Exercises: Outcrop and seismic sequence stratigraphy of carbonates

Application of carbonate sequence stratigraphy to rimmed shelves and to ramps.
Exercises: Seismic sequence stratigraphy of carbonates
Major Controls on Occurrence of carbonate platforms and genetic classification

Near-surface diagenetic environments of limestones and dolomitization, and relations to depositional sequences
Burial diagenetic environments of limestones and dolomites. Diagenetic trends.
Exercise: Diagenesis and porosity evolution of carbonates

Porosity and permeability evolution in carbonate reservoir rocks, rock types, and classification of carbonate reservoirs.
Exercise: Diagenesis and porosity evolution of carbonate reservoir rocks
Carbonate platforms, facies and reservoirs through geological time

Sequence Stratigraphy: Principles and Applications

This workshop presents the concepts and practical applications of sequence stratigraphy for petroleum exploration and production. All concepts are illustrated with field examples of seismic, well-log, core, and outcrop data. In-class exercises emphasize the recognition of sequence stratigraphic surfaces and systems tracts on well-log cross-sections, seismic lines, and outcrop profiles. The points of agreement and difference between the various sequence stratigraphic approaches (models) are discussed, and guidelines are provided for a standardized process-based workflow of sequence stratigraphic analysis. This enables the practitioner to eliminate nomenclatural or methodological confusions, and apply sequence stratigraphy effectively for facies predictions in exploration and production.

Daily Title: FUNDAMENTAL CONCEPTS OF SEQUENCE STRATIGRAPHY

o Topic 1: Stratal stacking patterns
o Topic 2: Shoreline trajectories
o Topic 3: Historical developments
o Topic 4: Workflow of sequence stratigraphic analysis

Discussion of stratal stacking patterns, which are key to the definition of all units and surfaces of sequence stratigraphy. Shoreline trajectories: forced regressions, normal regressions, transgressions. Exercise: Observation of stratal stacking patterns on 2D seismic data.

Daily Title: SEQUENCE STRATIGRAPHIC SURFACES

o Topic 1: Types of stratal terminations
o Topic 2: Sequence stratigraphic surfaces
o Topic 3; Other types of stratigraphic contacts

Definition of all sequence stratigraphic surfaces: subaerial unconformity, correlative conformity, basal surface of forced regression, regressive surface of marine erosion, maximum regressive surface, maximum flooding surface, transgressive surface of erosion. Distinction between sequence stratigraphic surfaces and other types of stratigraphic contacts. Exercise: Identification of sequence stratigraphic surfaces on a well-log cross-section.

Daily Title: SYSTEMS TRACTS

o Topic 1: Systems tracts in downstream-controlled settings
o Topic 2: Systems tracts in upstream-controlled settings
o Topic 3: Economic potential of all types of systems tract

Distinction between downstream-controlled setting (conventional sequence stratigraphy) and upstream-controlled settings which are beyond the influence of relative sea-level changes. Definition and examples of all types of systems tracts. Exercise: Identification of systems tracts on 2D seismic data and well-log cross-sections.

Daily Title: SEQUENCE MODELS

o Topic 1: Depositional sequences
o Topic 2: Genetic stratigraphic sequences
o Topic 3: Transgressive-regressive sequences
o Topic 4: Parasequences

The points of agreement and difference between the various sequence stratigraphic approaches (models) are discussed, and guidelines are provided for a standard process-based workflow of sequence stratigraphic analysis. This enables the practitioner to eliminate nomenclatural or methodological confusions, and apply sequence stratigraphy effectively for facies predictions in exploration and production. Examples of sequences in all depositional settings.

Daily Title: SCALE IN SEQUENCE STRATIGRAPHY

o Topic 1: The importance of scale: exploration vs. production development
o Topic 2: Sequence stratigraphic hierarchy
o Topic 3: Moving forward to a standard application of sequence stratigraphy

Discussion of scale, selected by the practitioner and/or imposed by data resolution, as key issue which determines the practical applications of sequence stratigraphy (e.g., petroleum exploration vs. petroleum production development). Latest developments in the definition of sequence hierarchy systems; definition of hierarchical orders. Exercises: Observation of sequence stratigraphic units and bounding surfaces at different scales.

Global Tectonics and Geological Prospecting Tools for Exploration

This hands-on 5-day course utilizes lectures and practical exercises to introduce key concepts of an effective petroleum system with emphasis on global tectonics and how basins are formed, filled and deformed, and on learning key geologic prospecting tools for exploration. The Atlantic Margin basin is used as an example to discuss the elements of the petroleum system. Clastic rocks are emphasized (especially turbidites) but carbonate rocks and evaporite formation is also reviewed.

Discussions include introduction of key aspects of source rock, migration, overburden rock, reservoir characteristics including porosity, permeability and architecture, seal rock characteristics, structural and stratigraphic traps, timing and preservation elements.

Prospecting Tools covered in this course include petrophysics for exploration geoscientists , bio- and sequence stratigraphic applications, essential log correlation workflow and log annotation methods, data contouring and mapping, applications of multiple working hypotheses, and utilization of direct hydrocarbon indicators to exploration. Students correlate various logs that illustrate faulted sections and straitigraphic variation, and create contour maps and learn to properly present faults in map view.

Objectives and Overview of Module and Course

Five Laws of Geology: Uniformitarianism, Superposition, Original Horizontality, Cross Cutting Relationships, and Walther's Law

Petroleum Systems – components, definitions, and assessment

Basin Formation

Plate Tectonics and the Wilson Cycle

Source Rocks in Conventional and Unconventional Petroleum Systems

Basin Filling – clastics

Carbonates and Dolomitization

Clastic Depositional Models - Fluvial

Clastic Depositional Models – Delta and Deepwater Turbidite

Walthers Law

Faulting as migration pathways and/or seals

Review of Fault Types

Hydrocarbon Traps - Good and Bad

Salt Deformation

Review of exploration in Salt Basins

Sutures and Inclusions

Rubble Zones and Imaging below Salt

Petrophysics for Exploration

Rock and Fluid Properties

Porosity and Permeability

Pressure and Compaction

Compartmentalization

Tool Applications

Biostratigraphy and Sequence Stratigraphy

Accomodation Space

Stratigraphic Sequences HST, FSST, LST, TST

Carbonate vs Clastic Sequence Stratigraphy

Seismic Sequence Stratigraphy

Log Correlation Purpose and Methods

Correlations for Structure

Geological Scenarios for correlation

What to Correlate? Log Headers

Facies Correlation and accounting for Faults

Using Seismic to aid correlation

Mapping and Contouring – Definitions and Principles

Creation of various types of contour map

Multiple working hypotheses

Contouring with Faults

Isopach Maps

DHI Concepts for Exploration

Seismic interpretation and Anomolies

Gas Effects and Bright Spots

Prospecting with DHI's

4D Seismic and Electromagnetic Data

Subsurface Facies Analysis - Integrating Borehole Images & Well Logs with Petrophysical and Seismic Data to Develop Geologic Models

This course has been designed for geoscientists, engineers and other technical staff who want to analyze and integrate image, log and dip data to enhance their understanding of exploration plays and field development. It leans heavily on worked class examples and case studies. Instead of interpreting image and dip data in isolation, the course shows how they can be used in conjunction with cores, other logs, modern depositional analogues, outcrop studies and hi-resolution seismic data to refine reservoir models.

Acquisition & processing, and structural analysis

Image & Dip Acquisition & Processing

- Measurement principles and wellsite acquisition

- Value of high resolution image data

- Image processing & display

- Dip computation and trouble shooting

Exercise with some real data

- Image & dip processing and LQC

- Image description & interpretation steps

- Comparison with core photos and description

Guide to Image Quality

- Exercises in bad borehole and tool responses

Structural analysis using image & dip data

- Structural dip trends and structural dip removal

- Unconformities

- Normal and growth faults

- Reverse and thrust faults

- Are faults sealing?

Sedimentology & continental settings

Stratigraphic analysis using image & dip data, 60 slides

- Depositional environments & facies analysis

- Lithofacies from log & image data

- Lithology, grain size variation, need to integrate

- Geometry

- Sedimentary structures

- Paleocurrent directions

- Integration & modelling at the field level

Eolian (wind-blown) sediments

- Sedimentary structures & dune forms

- Complexities in deposition setting & stratigraphic section

- Building reservoir model & populating with data

- Outcrop studies as input to reservoir simulation

Fluvial (river) sediments

- Fluvial settings (various models)

- Braided system lithotypes & sedimentary features

- Meandering system lithotypes & sedimentary features

- Point bar development (predictions)

- Channel models as developed by geostatistics

- Channel models constrained by outcrop analogues

- Correlation and sequence stratigraphic considerations

- Case study from Kalimantan; integrating high resolution seismic attributes with petrophysical data to fine tune a depositional model and site new wells; radically increasing oil recovery in the field.

Deltaic, coastal and shelf settings

Deltaic sediments

- Delta classifications and models

- Associated sand geometries

- Image & dip character in distributary fronts & channels

- Case study from South Sumatra basin; developing a play concept to identify most prospective area within structural closure

Coastal & shelf sediments

- Interrelation of coastal & shelf depositional settings

- Facies variation in prograding coastal sequences

- Idealized dip and grain motifs in bar/barrier sands

- Image & dip examples in shelf bar and barrier island sands

- Channel sands in a tidal setting

- Chasing channels by integrating image and seismic data

- Case study: distinguishing channel from bar sands in tidal settings, and its importance on reservoir characteristics

- Use of Nuclear Magnetic Resonance to distinguish sand units

- Using high resolution images to interpret thinly bedded reservoirs

Correlation Exercise

- Environments make a difference

Carbonate shelf & Deepwater

Sequence Stratigraphy

- Parasequences & basin margin architecture

- Aid to correlation & modelling

Carbonate shelf sediments

- Carbonate models and facies in coastal and shelf settings

- Carbonate reefs, and orienting reefal trends

- Porosity enhancement and reduction

- Sequence stratigraphy in carbonate sequences

- Generating reservoir model from outcrop data and 3D seismic

Case study: Kazakhstan Tengiz-Kashagan Trend

Case study: Carbonate rock typing; Corelab approach

Deepwater sediments

- Deepwater sediment depositional models

- Image & dip character in proximal & distal settings

- Orienting channel sands using image & dip data

- Using outcrop work for forward modelling to better interpret seismic, and understand reservoir production behavior

Case study: Improving log interpretation by modelling thin bed effects

Fractured reservoirs

Fracture systems

- Fracture types; open, healed, vuggy, syneresis

- Natural or induced; borehole breakout & tensile fractures

- Impacts on planning fracture jobs for stimulation

- Fracture orientation

Fractured Reservoir case studies

- Case study: Identifying & evaluating producing horizons in fractured basement offshore Vietnam

- Case study: Simulation of a producing fracture system in a Mid-East Giant

Geothermal systems in volcanic rocks (optional)

- Lithofacies in volcanic settings

- Case study: Using images to resolve reservoir delineation and development issues.

Geology