Weathering is an event that changes rocks and sediments when they are exposed to the Earth's surface conditions. This paper presents an ontological analysis and proposes a model for this type of event, based on the Unified Foundational Ontology and specializing concepts from the GeoCore ontology. The proposed model details the two main types of weathering processes (i.e. Physical Weathering and Chemical Weathering) and describes them in terms of transitions between situations, detailing the involved continuants and the properties that are affected, as well as how they are arranged in the situations connected by such processes. With this structure, the result of this work can help in tracing the past states of sediment and rocks before they are transformed into a weathering event and helps modelers to understand how to model physical occurents.
A good ontology model of events1 can aid in solving a variety of problematic tasks, such as dealing with temporal issues in information retrieval (e.g., in a large information repository which describes the states of a same entity in distinct moments, retrieve the currently valid state of a given entity) and processing or inferring past configurations of state of affairs based on the current state and a sequence of events that affected it. The oil and gas industry deals frequently with this kind of problem, on reservoir geology studies, and can benefit from accurate ontological models of geological events.
Geology, as most of natural sciences, is moving from an empirical approach of manually capturing and analyzing information to a more descriptive and quantitative model of work. The new approach demands a crescent digitalization of geological information, which requires an underline set of formal models for systematically acquiring and processing information. Moreover, the geologist activity is strongly based on abductive reasoning, trying to understand past events that bring the rocks and sediments to the current situation. The interpretation of these events allows geologists to create models of past environmental conditions and the evolution of them through time, which supports modeling the 3D distribution of reservoirs. Formal ontologies play a central role in this scenario, both for offering uniform representation of data as well as for allowing computer applications to simulate the interpretation reasoning of the geologist.
1In this work we will use the terms event, process, and occurrent interchangeably to refer to the things that happen in time.
Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
Sedimentology is a sub-field of the Geology domain concerned with studying sediments, sedimentary rocks, and the events they participate in the natural world. Sedimentary rocks are important for the economy, as they are bearers of oil and gas, and some ores (coal, placer diamond, and gold). Therefore, understanding the processes that create and modify sediments and sedimentary rocks is an important part of geologist activity.
Weathering is one of these sedimentary processes, and it congregates all chemical and physical processes that affect rocks and sediments due to the influence of the weather. Weathering is important as the first process at the beginning of the sedimentary part of the rock cycle. It is weathering that breaks igneous and metamorphic rocks into the particles that will eventually become a sedimentary rock. Weathering also modifies sediments and sedimentary rocks, becoming relevant for petroleum geology as this process can either enhance rock porosity, forming networks of highly permeable karsts, or close porosity, forming layers that will act as barriers for hydrocarbon flow.
In this context, this paper presents an ontological analysis and proposes an
ontological model for the process of weathering, covering its two main types: physical
weathering and chemical weathering. The model is based on the Unified Foundational
Ontology (UFO) [
The remainder of this paper is organized as follows: in section 2 we bring a brief introduction to the UFO and GeoCore concepts used; in section 3 we describe weathering from the domain perspective; in section 4 we present the ontological analysis and proposed model for the weathering event; in section 5 we discuss related work; finally, in section 6 we present the most relevant aspects brought up by this work and present our final remarks.
[
Concerning occurrent types, they gather things that unfold in time,
accumulating temporal parts, and that are existentially dependent on the objects that participate in
them. To account for this notion, UFO defines the concept of event as the transition from
an initial situation that triggers the event (e.g., some water exposed to below freezing
temperature triggers a freezing event), to an ending situation that the event brings about (e.g.
a wet soil after raining) [
In UFO, an event is also regarded as anything that happens to a selected set of
focal qualities in a particular spatiotemporal region [
The GeoCore ontology [
The GeoCore covers naturally existent material entities produced by a geological process. The main structure embodies the dichotomy between the independent continuants Geological Object and Earth Material. Geological objects are material entities with unifying principle and are constituted by Earth Material. Earth Material is an homeomerous amount of matter with no unifying or countable principles, including rock, mineral and fluid, such as water, gas and oil. Existentially dependent entities complete the ontology: Geological Structure (internal or external arrangement of objects), Geological Contact (surface shared by two geological objects), Geological Boundary (surface that delimits an amount of matter in an object) and Geological Age (geological age of creation).
Rocks and sediments are constituted by an aggregate of particles. When they are formed, the temperature and pressure conditions play a determining role in defining which minerals will constitute the particles. Most rocks are formed on pressure and temperature conditions very different from those happening at Earth’s surface. Frequently this mineral assemblage is chemically unbalanced with surface conditions. Therefore, while being exposed to surface conditions, the rock mineral composition might change and the bonds holding the rock’s particles together might break, forming new particles. This process of changing rocks, sediments, and their components due to Earth’s surface conditions is called weathering. The process of weathering is representative, in terms of requirements for ontology modeling, to most of the physic-chemical processes described in natural sciences. They request a set of conditions in the environment to happen and a trigger to start.
Weathering is divided in two kinds of processes, depending on the kind of change
on the involved entities. Physical weathering is the disintegration of the rock, breaking
it in smaller particles and resulting in the formation of sediments [
Physical weathering involves changing in the forces acting on rocks and
sediments, breaking the bonds that keep rocks and sediment’s particles indurated.
This might happen with the participation of water as in freezing-thaw weathering
[
Chemical weathering is a process that changes the chemical composition of the rocks or sediments. It happens due to the chemical unbalance of the minerals constituting a rock or sediment with meteoric waters near Earth’s surface. This leads to different reactions of water with specific minerals. It can cause the dissolution of some minerals, and formation of more stable minerals by hydrolysis, oxidation, hydration, dehydration, and other less common processes. The most common minerals appearing in chemical weathering are clay minerals and iron and manganese oxides. The changes in composition gradually reduce the consolidation of the rocks, assisting in the disaggregation of the rock.
This section presents an ontological model for the process of weathering. It first presents
the main types of continuants which are involved in this process. Then it brings a
description of the weathering process, detailing its main subtypes in terms of their triggering
and resulting situations, as well as their participants. The diagrams presented use the
OntoUML language [
The process of weathering is materialized by changes in the geological objects constituted by rocks and sediments, and as a result of the action of water, biological entities or solar radiation. Following we present the definition of the types of continuants used in this analysis (Figure 1).
GC:Earth Material is defined in the GeoCore ontology as gathering instances of natural amounts of matter which are generated by a Geological Process. GC:Earth Material has two specializations, GC:Earth Fluid (which is also part of the GeoCore ontology) and Earth Solid, gathering materials that naturally occur in fluid or solid state respectively. As these concepts are rigid but do not provide an identity criterion for its instances, they are classified into UFO’s Category.
Among the earth fluids we have Water, which we defined as a portion of a fluid
composed by a portion of the homonymous chemical substance and other molecular
quantities that may be dissolved in it. Given that water provides a principle of identity
criteria for their instances and has an individuation principle based on the notion of chunk
(i.e., A maximally self-connected portion of matter), it is classified as a UFO’s
Quantity [
Earth Solid is specialized into GC:Rock and Sediment. GC:Rock is
ontologically defined by the GeoCore ontology [
Both Sediment and GC:Rock are close to the idea of Amount of Matter in DOLCE
[
In this work we characterize Earth Material by its geochemical composition (GC) and mass, which are both UFO’s Qualities. GC represents the volumetric proportion of the different types of molecules in the entity. This quality is related to the minerals that constitute the particles forming an Earth Solid, and to the mixture of molecules in instances of Earth Fluid. Mass is a quality that corresponds to the homonymous physical property of an amount of matter (i.e., the quantification of the mass of an amount of matter). In addition, earth solids are characterized by their petrographic composition (PC) and particle size average (PSA). PC is a UFO’s Quality representing the volumetric proportion of the pores and of each kind of particles in an Earth Solid. PSA is also a UFO’s Quality, which represents the average size of the particles in an Earth Solid. Besides that, Rock Object is characterized by its fracture density (FD), a UFO’s Quality representing the number of fractures per volume of rock in the object.
The main interaction that happens during a weathering process is that of an Earth Fluid filling the pores of an Earth Solid, represented by a UFO’s Relator called pore filling. The infilling fluid plays the role of Pore Fluid (a UFO’s RoleMixin) or the more specific Pore Water role (a UFO’s Role) when such fluid is a portion of Water.
Besides earth materials, we also included geological objects in our model since they are the focal participants of the weathering processes (i.e., it is the involved geological objects that endure the process, preserving their identity despite the qualitative changes). GC:Geological Object is part of the GeoCore ontology and gathers instances of material objects constituted by some GC:Earth Material. We specialized it into the Geological Solid Object, which gathers instances of GC:Geological Object constituted by some Earth Solid. As both of these types are rigid but do not provide an identity criterion for its instances, we classified them as UFO’s Category. On top of that, we define Rock Object and Sediment Object as a Geological Solid Object constituted by GC:Rock and by Sediment, respectively. These are rigid types that represent functional complexes into which rock and sediment amounts are arranged (e.g., geological layers, various types of sediment deposits) and that provide identity criterion for its instances, so that they are classified as UFO’s Kind.
It is noteworthy that in this work we use the notion of constitution described in
[Garcia et al. 2019], which is based on
Apart from earth materials and geological objects, Biological Entity type gathers
living things or its parts, including entities with diverse principles of identity such as roots
of plants or any burrowing and sediment feeding animals. Thus, it is also a rigid concept
that does not provide an identity criterion for its instances, being classified as a UFO’s
Category. Completing the inventory of continuants, Solar Radiation gathers portions of
energetic radiation, constituted by photons and other particles emitted by the Sun and we
classified it into UFO’s Quantity. [
As previously described, Weathering is a process that involves the interaction of an Geological Solid Object and some environmental element that results in change of characteristics or the creation/destruction of a Geological Solid Object. It is triggered by a Geological Solid Object exposed to surface conditions situation and brings about a Weathered Geological Solid Object situation.
There are a variety of classification criteria according to which we can specialize
types of occurrent [
Both mechanical and chemical qualities may be affected in a single occurrence of Weathering. In such cases, Weathering unfolds as a complex event, composed of simultaneous subprocesses of Physical Weathering and Chemical Weathering (Fig. 2B). With that, the triggering and ending situations of such occurrences are composed of the triggering and ending situations of its subprocesses. These subprocesses and their initial and ending situations are detailed in the following sections.
In a Physical Weathering event (Fig. 3A) a Geological Solid Object is mechanically fragmented due to physical stresses associated with surface environmental conditions. Thus, this event is triggered by a situation of Geological Solid Object exposed to physical stress due to surface exposition, that mandatorily has the presence of the Geological Solid Object that will be broken into smaller pieces. In addition, Water, Biological Entities, and/or Solar Radiation are also present when they act as the agent changing the forces in the object, but they suffer no change in its properties during the event. The Physical Weathering event brings about a Broken-down Geological Solid Object situation containing the instances of Geological Solid Object modified or created during this process. Physical Weathering can also be specialized into Rock Physical Weathering and Sediment Physical Weathering (Fig. 2A), according to the type of Geological Solid Object present in its initial situation (i.e., rock or sediment object).
A Rock Physical Weathering (Fig. 3B) is triggered when the Geological Solid
Object that is present in the initial situation is a Rock Object, being a Rock exposed to
physical stress due to surface exposition situation. This event provides the historical role
of Source Rock for the instance of Rock that constitutes the Rock Object that is suffering
the weathering. It is important to make clear that, though being referred by the same term,
this role has nothing to do with that of a rock in a petroleum system as the generator of
hydrocarbons [
A Rock Physical Weathering might lead to three different resulting situations depending on the intensity of the physical stresses applied in the rock, the duration of the event and the fragility of the rock. The first possible outcome is a Broken-down rock situation, in which the initial rock object is fractured, being broken in several pieces (though it remains as the same, single object). The Rock Object in the initial and final situations have the same PC and GC value. However, the new fractures on the final rock object increases the value of its FD. This case is exemplified in the Fig. 3D.
A second possible outcome of a Rock Physical Weathering event is a Brokendown sediment situation, in which a single instance of Sediment Object is present. In this case, the rock object in the initial situation is broken in so many small parts that it can no longer be regarded as a rock object, but rather the aggregate of such little parts constitutes a new amount of Sediment, which constitutes the new Sediment Object that is created during the event. This Sediment Object has the same GC value as the initial rock object, as its particles are made of the same minerals, but it has a different PC value, as the proportion and kind of pores and particles is altered with the fragmentation. The mass value of the initial rock instance and the final sediment instance are the same.
The third possible outcome of a Rock Physical Weathering event is a Broken-down sediment and rock situation, in which an instance of Sediment Object is created and the Rock Object instance remains. Fig. 3E exemplifies an occurrence of a Rock Physical Weathering bringing about this type of situation. In the situation described, the sediment and the rock objects have equal GC value as the initial instance of Rock Object, but the Sediment Object created has different PC value. The mass values of the sediment and rock objects are both lesser than the previous mass value of the instance of Rock Object (since the new object result from the fragmentation of part of the initial one).
A Physical Weathering event can also be specialized in a Sediment Physical Weathering event (Fig. 3C). It is triggered by a Sediment exposed to physical stress due to surface exposition situation, in which an instance of Sediment Object is present.
In this event, the particles that form the sediment that constitutes the object are mechanically broken into smaller particles, bringing about a Broken-down sediment situation at the end. The instances of Sediment Object in both initial and final situations are the same, with a lesser PSA value at the end. The PC value of the sediment object might change or not, depending on whether or not the break-up of the particles changes their type, but the GC and mass values remain unaltered. An instance of this event is represented in Fig. 3F in which only the PSA value of the Sediment Object instance is changed by the event.
A Chemical Weathering event is triggered by an Geological Solid Object infilled with Meteoric Water situation (Fig. 4A), in which an instance of Geological Solid Object is infilled by an instance of Meteoric Water (which plays the role of Pore Water). Then, it brings about a Chemically altered Geological Solid Object situation with the presence of the same instance of Geological Solid Object and a new instance of Meteoric Water. During this event the Geological Solid Object is submitted to a chemical unbalance with the water filling its pores, resulting in exchange of mass between the Geological Solid Object and the Meteoric Water. This is materialized by a change of GC, Mass and PC values of the participants in the initial and final situations. An instance of this event is represented in Fig. 4B.
Since petroleum reservoirs are in subsurface and data is scarce for direct analysis, geoscientists need to propose models to the distribution of rock bodies. The traditional approach is building statistical models that describe the geometry and distribution of rock bodies, associating them to flow properties in order to evaluate the production capability of wells. However, this method may hide important properties of reservoirs concerning spatial disposition evaluation and internal barriers for flow, leading to production misevaluation. Thus, recently, efforts have been directed to complement the quantitative approaches with knowledge-based models, including ontologies.
An example is the work in [
An account of the process of weathering can be found on EcoLexicon
(ecolexicon.ugr.es), a multilingual thesaurus specialized in the environmental domain. It is
structured as propositions and knowledge frames that are organized in an ontological
structure, which focuses on perceptual information and semantic relations [
As pointed out in [
This paper presented an ontological analysis and the corresponding model of the process of weathering, which was developed based on Unified Foundational Ontology (UFO) and GeoCore Ontology. The model details the two specializations of this event, namely the physical weathering and chemical weathering, specifying their initial and ending situations as well as the involved participants and their affected properties.
By defining the weathering process in terms of its initial and ending conditions, the presented model can help in describing the chain of events that led the rocks and sediments to their current situation. Moreover, by indicating the observable properties that are changed in weathering processes, the model may allow integrating the data collected about a region of interest into the context of these occurrences. In fact, by defining the process as the change on properties of its participants or their creation/destruction, we add a new layer of meaning in relation to what was done in previous work, which can offer useful insights for those who deal with modeling and simulating events in related natural domains, such as sedimentology processes. Furthermore, this enriched model may improve the understanding of sedimentary occurrences where the data is incomplete, leading the geologists to fill the gaps of information on the geology records, providing a more complete view of the exploration area.
Besides that, by proposing the physical weathering event as a truthmaker for the source rock historical role, the model also contributes to provenance study, i.e., a branch in geology that aims at identifying the rocks from which the sediments originated. In this context, the weathering model can be integrated with other event models to rebuild the full path of sediments. Tracing the source rock of a weathering event that created sediments can also be directly useful in reservoir characterization, since they reveal characteristics that influence the quality of the reservoir.
Given this structure, we believe that this process modelling proposal can offer useful insights for those who deal with modeling and simulating events in related natural domains, such as sedimentology processes. Nevertheless, the model presented here is the foundational stone for modelling this geological process and does not exhaust the theme. In the current state, the model does not include explicit representations for some environmental aspects that affect the entities involved in weathering processes. For example, it does not include temperature and external pressure which are needed to represent insolation and stress-release weathering, nor does it cover the specific interactions involving biological entities. Besides that, the model can be improved by a clearer ontological account of the notions of rock and sediment (since they don’t seem to fit any of the metatypes provided by UFO), as well as their relation with the objects they constitute, which are in progress within the research group.
Finally, future work will focus on detailing more specific types of weathering (e.g., freezing-thaw, wetting and drying, insolation, stress-release) as well as classifying it into more general types of process (e.g., natural process, creation, destruction). Additionally, we will further the modeling to encompass weathering processes in smaller scales (e.g., the effects over grains of sediment). Another horizon of investigation is the uncovering of the dispositions involved in the triggering of weathering processes in order to better support reasoning and simulation tasks.
The first author thanks Petrobras for supporting this work. This work was financed in part by the Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de N´ıvel Superior - Brasil (CAPES) - Finance Code 001, and by the Brazilian Council of Research (CNPq).