With reference to crude petroleum composition, which class of hydrocarbons is almost absent in native crude oils before refining?

Introduction:
Crude oil is a complex natural mixture dominated by paraffins and naphthenes with varying aromatics and sulphur species. Understanding which families are naturally present guides processing expectations.

Given Data / Assumptions:

• We consider native crude (pre-refining) composition.
• We identify the class nearly absent in untreated crude.

Concept / Approach:
Olefins (alkenes) are typically reaction products of cracking and are rarely present in significant amounts in native crude because they are less thermodynamically stable than paraffins/naphthenes over geologic timescales.

Step-by-Step Solution:
1) List natural families: paraffins, naphthenes, aromatics; sulphur compounds including mercaptans may appear.2) Recognize olefins form mainly during thermal/catalytic cracking, not in the reservoir.3) Conclude olefins are almost absent in crude petroleum.

Verification / Alternative check:
Analytical surveys of crude oils confirm negligible olefin content; refinery olefins mainly originate downstream in cracking units and steam crackers.

Why Other Options Are Wrong:
Mercaptans: Frequently present in sour crudes in small to moderate amounts.Naphthenes/cycloparaffins: Common and often abundant.Paraffins: Common and often dominant.

Common Pitfalls:
Assuming refinery product distributions reflect reservoir composition; cracking products like olefins largely arise during processing, not in situ.

Final Answer:
Olefins (alkenes)