Amplitude Anomaly: Definition, DHI, and Hydrocarbon Indicator

A amplitude anomaly is an abrupt departure from the background amplitude level observed in a seismic dataset that cannot be explained by structural, stratigraphic, or processing artifacts alone. In practice, the term is used almost exclusively for anomalies that serve as direct hydrocarbon indicators (DHIs): seismic amplitude signatures that result directly from the presence of gas, oil, or gas-oil or gas-water contacts in the pore space of a reservoir rock. The three canonical forms of DHI amplitude anomaly are the bright spot (anomalously high amplitude), the dim spot (anomalously low amplitude), and the flat spot (a horizontal reflection cutting across structural dip, marking a fluid contact). A fourth type, the polarity reversal, marks the transition from a positive-polarity reflection above a gas-water contact to a negative-polarity reflection below it, or vice versa, depending on the impedance contrasts involved. Collectively, these four expressions make amplitude anomaly analysis the most direct and cost-effective method of hydrocarbon detection available before drilling, though no DHI is infallible without well calibration.

Key Takeaways

• An amplitude anomaly is a significant departure from background seismic amplitude that may indicate a direct hydrocarbon indicator (DHI), most commonly a bright spot, dim spot, flat spot, or polarity reversal.
• Bright spots occur where gas or light oil lowers the acoustic impedance of a sand below that of the encasing shale, increasing the negative reflection coefficient at the sand top and producing a high-amplitude trough on normal-polarity data.
• Flat spots are horizontal reflections within an otherwise dipping structural section, indicating a gas-water, oil-water, or gas-oil contact; their horizontal geometry is the most geologically unambiguous of all DHI types.
• AVO analysis extends amplitude anomaly detection by measuring how amplitude varies with source-receiver offset, providing a gradient attribute that discriminates gas-filled sands (Class III) from lithology-only anomalies and from Class I hard-kick sands.
• Tuning at the quarter-wavelength thickness and seismic multiples are the two most common causes of false amplitude anomalies; calibration to well data is mandatory before using an amplitude anomaly as the primary evidence for a drilling commitment.

How Amplitude Anomalies Form: Rock Physics of Gas and Oil Sands

The acoustic impedance (Z = density x P-wave velocity) of a rock changes dramatically when its pore fluid changes from brine to gas. Gas has a very low bulk modulus (high compressibility) compared with brine or oil. When a gas replaces brine in a porous sand, Gassmann's fluid substitution equations predict a sharp decrease in the rock's bulk modulus and a correspondingly large decrease in P-wave velocity. The density of the rock also decreases slightly because gas is less dense than brine. Both effects reduce acoustic impedance. If the encasing shale cap has a higher impedance than the gas sand, the reflection coefficient at the top of the sand is negative (shale impedance minus gas sand impedance yields a negative value), producing a trough on normal-polarity seismic data. Because gas lowers impedance more strongly than brine, the absolute value of the reflection coefficient increases with gas saturation, increasing the amplitude of the trough. This high-amplitude trough is the bright spot. The same mechanism applies to oil sands, but to a lesser degree: oil is denser and stiffer than gas, so the impedance contrast is smaller and the bright-spot effect is weaker. Light oils at high reservoir pressures and moderate temperatures can still produce measurable bright spots, while heavy oils may increase impedance and produce the opposite effect.

The relationship between gas saturation and velocity is highly nonlinear. At even low gas saturations of 5 to 10%, the P-wave velocity of a porous sand drops nearly as much as it does at full gas saturation (100%). This means that a small gas cloud can produce a bright spot that looks similar to a large gas accumulation on amplitude maps. Conversely, the S-wave velocity (shear-wave velocity) is insensitive to fluid type (fluids have no shear modulus), so the ratio of P-wave to S-wave amplitude versus offset (AVO) provides additional discrimination. The Vp/Vs ratio decreases strongly in gas sands (Vp falls, Vs stays approximately constant), producing distinctive AVO anomalies that complement the amplitude bright-spot observation.

The Three Main Amplitude Anomaly Types

Bright Spot

A bright spot is an anomalously high-amplitude reflection that exceeds the local background amplitude by a factor typically exceeding 1.5 to 2.0 (the threshold is survey-dependent and defined relative to the amplitude of nearby non-anomalous reflectors of comparable depth). Bright spots are most diagnostic of gas when they: (1) conform to structural closure (the amplitude high coincides with the structural crest and fades toward the flanks at a consistent depth); (2) have reversed polarity relative to the seafloor reflection (indicating a soft top); (3) are accompanied by a flat spot at the base; and (4) show an AVO Class III response (amplitude increases with offset). In the Gulf of Mexico Miocene, the bright spot was the primary exploration tool from the 1970s onward, and operators such as Shell, Exxon, and Chevron drilled hundreds of wells based primarily on amplitude conformance mapping before AVO analysis became standard. The success rate for structural amplitude conformance plays in the Miocene GOM was approximately 60 to 70% in prolific areas, vastly better than the 20 to 30% success rates achieved in pre-amplitude exploration.

Dim Spot

A dim spot is an anomalously low-amplitude or absent reflection over a structural closure. It occurs when the gas sand or reservoir has a higher acoustic impedance than the encasing shale rather than lower (a "hard" reservoir), so that the presence of gas decreases the reflection coefficient rather than increasing it, reducing amplitude. This is the Class I AVO sand scenario. Examples include tight carbonates and highly cemented sands in which the frame stiffness dominates the acoustic impedance and gas saturation causes only a moderate velocity decrease. Dim spots are more ambiguous than bright spots because a low amplitude could also reflect poor seismic data quality, structural complexity, or a water-bearing sand with similar impedance to the cap rock. The supporting criteria for a dim spot as a genuine DHI include structural conformance of the dim zone, a flat spot at the predicted contact, and a Class I AVO response (amplitude decrease or polarity reversal with offset).

Flat Spot

A flat spot is a horizontal reflection within a seismic volume that cuts across structural contours. Because fluid contacts (gas-water, oil-water, gas-oil) are controlled by gravity and buoyancy forces and are therefore horizontal (parallel to sea level), a reflection from a fluid contact will be exactly horizontal even in a steeply dipping reservoir structure. This horizontal geometry is the most geometrically unambiguous of all DHI indicators: no stratigraphic or structural artifact naturally produces a perfectly horizontal reflection cutting through dipping reflectors. The flat spot reflection arises from the impedance contrast between gas-saturated rock above the contact and brine-saturated rock below. At the gas-water contact, the lower layer is brine sand (higher impedance) below gas sand (lower impedance), producing a positive reflection coefficient (a peak on normal-polarity data). The flat spot peak, sitting just below the negative trough of the bright-spot top-of-gas reflection, is the classic DHI pair that provides the highest possible pre-drill confidence in an untested exploration prospect. The absence of a flat spot in a putative bright-spot play increases uncertainty: the sand could be fully gas-saturated (column extends below seismic resolution), the contact could be below the lower frequency resolution limit, or the bright spot could be a lithology effect without free gas.

AVO Classification and Its Role in Amplitude Anomaly Interpretation

AVO (Amplitude Variation with Offset) analysis, developed systematically by Ostrander (1984) and extended by Rutherford and Williams (1989), classifies gas sands into four types based on how their normal-incidence reflection coefficient and their amplitude-versus-offset gradient relate to each other. This classification is essential for understanding which type of amplitude anomaly a given geological setting will produce:

• Class I: The gas sand has higher acoustic impedance than the encasing shale. The reflection from the top of the sand is a positive peak (hard kick) at normal incidence, and amplitude decreases with offset, potentially crossing through zero (a polarity reversal) at intermediate offsets. The anomaly at the stack level is a dim spot or polarity reversal rather than a bright spot. Examples: Cretaceous tight sands in the North Sea, some carbonate-cemented sands.
• Class II: The gas sand has acoustic impedance approximately equal to the encasing shale. The normal-incidence reflection coefficient is near zero, so the event is almost invisible on near-trace stacks. Amplitude increases with offset (in absolute value), producing a strong far-offset anomaly. This class is divided into Class IIp (positive normal incidence RC, amplitude dims then reverses) and Class IIn (negative normal incidence RC, amplitude grows negative with offset). Class II anomalies are particularly dangerous for explorers using only full-stack data because the amplitude anomaly is subtle or absent.
• Class III: The gas sand has substantially lower acoustic impedance than the encasing shale. The top-of-sand reflection is a large negative trough at normal incidence (bright spot), and amplitude increases (becomes more negative) with offset. This is the classic bright-spot DHI of the deepwater GoM, Paleogene turbidites, and the Cenozoic sections of the Niger Delta, Nile Delta, and offshore Brunei. Class III is the most reliable DHI because both the stacked amplitude and the AVO gradient independently indicate gas.
• Class IV: The gas sand has lower impedance than shale (like Class III, so a bright spot at normal incidence), but amplitude decreases with offset (opposite to Class III). This unusual behavior occurs when the velocity contrast is positive even though the impedance contrast is negative, a situation possible at shallow depths or with specific mineralogy. Class IV anomalies can be confused with Class III unless offset-dependent amplitude analysis is performed.

In deepwater GoM exploration, Class III anomalies in Miocene and Pliocene turbidite sands account for the majority of historically drilled DHI plays. In the Norwegian North Sea Paleocene Heimdal sandstone play, Class III bright spots guided major discoveries including Balder and Grane. In contrast, many onshore North American plays involve Class I or II sands where the bright-spot DHI concept does not apply and AVO gradient analysis is required to detect hydrocarbons.